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You can buy Mythic Adventures, which is a Paizo source (available from Lone Wolf for HL).

You can also buy Mythic Hero's Handbook, Mythic Spell Compendium, and Mythic Monster Manual (although the latter might not be for a player); all of these are done by Legendary Games and you can buy them from Hero Lab, through their store.

The Mythic Hero's Handbook has a more complete Psionic system, but there was something in the Community Pack for mythic that sort of works in a similar manner.

Legendary Games released a couple more mythic books: Mythic Character Codex and another Mythic Bestiary, but there are not Hero Lab files for those.

If your group uses the Spheres of Power/Might system, there are mythic books for those (but they're based on Spheres of Power, not the revised Ultimate Spheres of Power, so will be glitchy phrasing/references at times, but they're still workable).

They are: Mythic Champions of the Spheres, Mythic Spheres of Power, Mythic Spheres of Might.

I allowed my players to also use Mythic Paths of the Lost Spheres, which was five or six additional mythic paths, as the balance was comparable to the Sphere mythic stuff (it is not Spheres of Power/Might related, despite the name) and to the Legendary game stuff.

Pretty much, check with the DM to determine the sources that are okay.

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There is a Gestalt addon, done by the community, which you probably have if you're experiencing the incorrect Path of War calculations.
I don't use the addon (mostly), but have it for the adjustments.

The gestalt method basically has Gesalt as a Class (counts as one class level), then a configurable tab where you choose your two classes, which is then three classes per character level.

I'd personally ditch the Gestalt addon for Hero Lab and use adjustments instead.

Instead of going:
Class 1 = Gestalt (Stalker + Fighter, in the configurable tab).

I just add Stalker and Fighter, and Hero Lab essentially shows two classes (and two levels taken).
This semi-breaks with the +1 ability adjustment per four levels, as you're going to get roughly twice as many as you should...
You can ignore the excess, or switch your stat generation to Pick/Choose, reduce a stat like Charisma by -1, then give the (per 4 level stat increase (that your shouldn't have) to Charisma, or whatever stat you lowered).

When I do Gestalt, I base it off of the side bar of 3.5e's Unearthed Arcana, page 73, which talked about the Fractional BAB and Fractional Saves.

Essentially, you take the best BAB from each class and use that:
+20 BAB at level 20, adds 1.00 (per level)
+15 BAB at level 20, adds 0.75 (per level)
+10 BAB at level 20, adds 0.50 (per level)

Take the better of the two for each level, and keep a running total.

On the Adjust Tab, if you have the community package & gestalt packages, you can type in Gestalt and you get two useful adjusts.

I add Gestalt Saves three times and Gestalt BAB once (as adjustments).
Then set the saves to Fort, Ref, and Will.
Generally the type is untyped penalty.
If you want the adjustments hidden, you can do them on the Personal Tab instead of the Adjust Tab, but I generally like having a lot of the Personal window viewable for notes.

Saving throws follow two progressions, either good or poor.
If it is good, then you're adding 0.50 (they use 06/12ths in Unearthed Arcana) per level.
If it is poor, then you're adding 0.33 (they use 04/12ths in Unearthed Arcana) per level.

Also if your save is good at character level one (so only as a level one character), you get a +2 bonus.

I'll note that the player has Barbarian | Inquisitor, which is good Fort, good Will and poor Ref.
For their adjustments, I note under Fort in the description text: 02 (06/12)

If checking the box shows a number higher than 02 (I apply a penalty) or lower than 02 (I apply a bonus).
When they level up, if either class at 2nd level has good Fort, I'd add 06/12 and make it 03 (00/12) on the description.
If neither class at level 2 has a good Fort, I'd add 04/12 and make it 02 (10/12) on the description line.
Then do whatever adjustment was necessary for the base save to match whatever the fractional save should be.

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With the Path of War, there is a useful adjustment for that too.

Basically go (in HL) to File > Print Preview...
Then select (the Dossiers button) Path of War Maneuvers (Descriptions).

It will output your Path of War stuff, but at the top it will say Stalker 4 (or whatever class and initiator level it is calculating).

You're supposed to have +1 per level in the actual class.
Or per level in a class like Awakened Blade, which advances the base class for maneuvers and initiator level.

You're supposed to only have +0.50 per level in other classes.

If you went:
01 Stalker 01 || Fighter 01
02 Stalker 02 || Fighter 02
03 Stalker 03 || Warlord 01
04 Stalker 04 || Warlord 02

You would want to calculate the Stalker as having 04 initiator levels, as the Fighter and Warlord contribute nothing (their 0.50 (that HL is giving) or more if you're also getting 0.50 from Gestalt (the class) is less than the 1.00 that you get for each Stalker level).

The Warlord though gets 0.50 from each of the Stalker/Fighter levels for +1.00 (from the two) and +1.00 from each of the Warlord levels for a total of 3.00.

If you calculate your Stalker level (Initiator Level) as 4.00 and your Warlord (Initiator Level) as 3.00, you can apply the adjustment "Initiator Level" on Adjust Tab, Other Adjustments as necessary to reduce what the PDF displays as Initiator Level (at the top) to what your initiator level actually should be.

I've noticed at higher levels the calculations for the Path of War stuff breaks, and it will eventually tell you you're not high enough level for anything that you have.
That is typically towards 15th level in two classes.

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If you're using Ultimate Spheres of Power and/or Spheres of Might, in a Gestalt game.

You can do the adjustment for Gestalt BAB.
And immediately then type in "Spheres" and add "Combat Training: Effective BAB".

You need to set that to a penalty type like Profane Penalty for it to actually count.
But just have it match exactly the adjustment on the Gestalt BAB adjust.
Might doesn't read off of the BAB you've done with Gestalt BAB adjust, but it will off of the Effective BAB adjust.

With Might, some spheres add a bonus rank per talent/sphere (up to 5 for the sphere, then 10 for the first talent in the sphere, 15 for the 2nd etc).
I just do an adjust with a skill penalty, because a level one character should cap at one rank but the addon is hardcoding two ranks for being a Stalker || Fighter.

Some things then calculate per six ranks (or four whatever) in a skill.
And you just have to manually note (I usually use the Personal Tab) that Athletics (Mobility) should be +3 not the +4 it is giving.