Gestalt - Community Package....

[Ualaa]

Is there an equivalent to the Player Pack, for the d20 System?
Or someone who has done content shared with the community?

If so, that would be your best bet.

Either that, or dive into the world of the Hero Lab Editor.
Generally people suggest finding something similar and copying that mechanic.
I could be recalling wrong, but are there classes like Mystic Theurge/Ur-Priest for 3.5, where your one level advances features from more than one class?
Something like that, might be the basis for copying the mechanic.

Probably not easy or fast.

.
.
.

A quicker way to do it, may be to simply take levels in both classes on top of each other.

With gestalt, it is suggested that you use fractional BAB, fractional Saves, and best progression for features that both sides of the gestalt share.
As per the side bar, within the Gestalt rules section of the Unearthed Arcana 3.0 book.

For fractional BAB, compare your two classes.
If either has full BAB (+20 to hit, at 20th level) advance your BAB by +1.00 for that level.
If either has 3/4 BAB as the better of the two classes (so +15 to hit, at 20th level), then advance your BAB by +0.75, for that level.
If both classes of the Gestalt only have 1/2 BAB (so +10 to hit, at 20th level), then advance your BAB by +0.50, for that level.

.
.

With fractional saves...
If either of your level one classes has strong saves +2 at first level... then they effectively have +2.5 (which on the 1/12th's scale is 30/12).
If between both of your classes, at first level your saves are poor (+0), then you effectively have (on the 1/12th's scale 4/12).

After first level, every time your character levels up, compare both classes.
If the saving throw is a good progression for that character, then advance that saving throw by +6/12th's.
If both sides of the gestalt have a poor progression for that specific save, then advance it by +4/12th's.

.
.
.

Best progression of abilities is basically Rogue sneak dice.
The rogue gains +1d6 sneak, at every odd level, and eventually hits +10d6 sneak dice at 19th character level.
If you find a class that also adds sneak dice, your character cannot have a better progression than the better of the two progressions.

In Pathfinder a rogue gains +1d6 (per two levels).
And in Pathfinder a slayer gains +1d6 (per three levels).
No matter how you stagger your levels, so 1d6 is gained as often as possible...
The better progression of the Rogue... with 1d6 per odd level means that at 9th level the character cannot have more than 5d6 sneak dice.
So a Rogue 10 | Slayer 10 has 5d6 sneak, not 5d6 from Rogue and 3d6 from Slayer for 8d6 total.

.
.
.

If you were to just add class levels, so if your desired Gestalt was Druid | Rogue.
At first level take Druid 1 and Rogue 1, being a 2nd level character as far as Hero Lab is concerned...

With fractional BAB, your character should be +0.75 (rounded down to +0, but record 0.75 somewhere)... and give yourself +2.5 (30/12) to each save, as the Druid has strong Fort/Will, while the Rogue has strong Ref.

For other features there isn't overlap.

The rogue has 8+INT skills, which is superior to the druid's.
The rogue has +1d6 sneak attack, and gains Trap Finding etc.
The druid has spellcasting as a first level druid.

For hit points, your character should have 8 (maxed at first level), plus (or minus) CON modifier.

Having given your character both a Rogue and a Druid level, you'll likely have too high in a few things... BAB, Saves, Hit Points, Skill Points (max skill ranks).

But you can adjust those, via adjustments.
And reduce your Gestalted character to the value that they should actually have.

So total BAB is +0.75. Fort/Ref/Will are all +2.50. Hit Points are 8+CON. Max ranks in a skill is 1, with 8+INT total skill ranks.
Adjust your numbers manually.

You'll have the features of both classes, because you physically added both within HL.

.
.
.

I don't possess ranks in Editor-fu, but that's how I would achieve Gestalt in Hero Lab, lacking a Gestalt package to utilize.