To be honest, nothing is technically necessary to make GVP compatible with E&C. GVP only adds new game elements in new files, so nothing will be in conflict unless the author of E&C for some reason went out of the way to deliberately override a GVP game element (which I doubt has happened!). The two mods will run together just fine as-is. The issue for "compatibility" is really more conceptual: do all the elements fit together in a way that makes sense, and if not, how can we make them make sense? It's a hard question to answer. E&C is a very big mod that adds and touches a lot of game elements. I have very little time for modding anymore, so a comprehensive patch that makes sure that, say, GVP civics logically take into account the ethics added by E&C is out of the question for me (but if someone else wants to try, go right ahead).

Another issue is content (specifically, governments) in GVP that E&C prevents from forming due to new requirements. For example, if a GVP government requires two civics that E&C makes incompatible with each other, then that government cannot be formed. Alternately, an E&C government might be weighted higher than a GVP government with the same requirements, which would make it so the E&C government always forms instead of the one from GVP. (Or, vice versa!) There's no clear "right" answer for how to address these cases because undoing or changing an E&C requirement to make GVP content appear might mean breaking something else in E&C. It's likely unavoidable that some governments added by GVP may not be possible to form with changes made by E&C. This does not effect the game-mechanical "meat" of GVP - the civics - and GVP adds an awful lot of governments that will probably mostly be unaffected, so I consider it an acceptable "loss."

The main thing I want to avoid is the player seeing silly government combinations generated by the game, e.g., Fanatic Competitive (read: capitalist) Workers' Councils, etc. Using the random_weight and ai_weight filters on civics, I have tried to make sure no randomly generated/AI-player-controlled governments will have combinations like that. I don't know if it works because again, very little time, but if you see some goofy combo of GVP and E&C elements, let me know in a reply here. Maybe it's something I haven't thought to check for! But to be clear, if you do see something like what I have described here, nothing is actually "broken" besides, perhaps, your immersion.

Note that this has nothing to do with what you, the player, can do. You can combine Cooperative (i.e., socialist) ethics with Enlightened Liberalism if you want, for example, and it's up to you to decide if that breaks your immersion. Controlling for this would require a patch, which (again) I don't have time to maintain.

As a final note, if you are not using E&C, you will see error messages in the log files complaining about not being able to find various ethics. That's normal; an unavoidable result of referring to content in a mod that hasn't been loaded.