Here's some helpful information from the community wiki:
https://github.com/Sklore/HL_DD_5e_Colab/wiki/Spells
The quick/dirty way to do this is to add the tag sClass.cHelpWiz to your "druid gnoll" to get all the wizard spells and do the same for any other class spell lists. You should already see that the "druid" gnoll has the sClass.cHelpDrd tag from where you gave it druid levels. This option gives you every spell and makes choosing spells a bit more cumbersome since the selection lists grow large.
Option 2 would be to add a ClsAllowSp.spIDXXXX tag for each spell that is not normally in the spell list assigned to your race. (Where spIDXXXX is the spell ID, like spFly, spCureWoun, etc.). This takes time though.
Both these options assume you're making a gnoll with the editor and not just in the HL client as a custom monster or adding classes to the monster.