Setting up a few mods for a friend's server.
Haven't edited any config so far.

I was just testing if everything was stable, going up to a Silkie made me crash.
Crash log right here

Edit: using last version of GoG (1.6.5 atm of writting) and Forge last stable (2768).

I guess the last update 1.6.6 fixes this since it got pushed out after your answer ?
Unrelated but, do I need to create a new world to benefit from GoG content ? Because I had to remove it from our server because of the crash and we began playing without it.
We definitely want to play with as we are old timers who used the very first versions of GoG back 2012/2013 and we loved it, getting to see it updated for 1.12.2 definitely took a jab at our little hearts, in a good way ! So even if we need to create a new world, we will.

Edit: and above all, thanks for answering and addressing this so fast !

I'm currently working on a possible solution to this issue. It should be up in a few hours after this post.

I'm not sure if it fixes it, but it should. I missed making additional changes when I modified the EntityAIGaiaAttackRangedBow (which seems to be the cause) in the last update.

No, you don't. GoG does not have any world-gen content (such as structures and blocks). Removing it does not leave behind any scars, so adding it back in should not be an issue. I'm glad to learn there are a group of people who played the very first version continue to play it to this day.

I had to. I can't sit here idly knowing that there is a major bug in the recent build. I would have addressed it earlier had I learned the issue was caused from the mod (due to the ticking entity issue being a common bug associated with dedicated servers).

Seems there's another crash with Minotaurus this time.
https://gist.github.com/Excaliburning/fb61221aee6585d1e0f8b6d105ef854c

:(
Thanks for being so active anyway !

This seems to be caused by EntityGaiaMinotaurus.isTarget() being declared as "client side only" when it clearly should not be.

Moreover, there seems to be no real reason to use @SideOnly for custom functions at all. From https://mcforge.readthedocs.io/en/latest/concepts/sides/:

Annotating a method or field with the @SideOnly annotation indicates to the loader that the respective member should be completely stripped out of the definition not on the specified physical side. Usually, these are only seen when browsing through the decompiled Minecraft code, indicating methods that the Mojang obfuscator stripped out. There is little to no reason for using this annotation directly. Only use it if you are overriding a vanilla method that already has @SideOnly defined. In most other cases where you need to dispatch behavior based on physical sides, use @SidedProxy or a check on getSide() instead.

Thanks for clearing that up; I was not sure how @SideOnly worked. I only added it because isSwingingArms had it.

isTarget() itself currently serves no use. I was planning to use it to fix the issue of mobs attacking a target despite the target no longer being there. I guess I will be removing it in the next update and hopefully then all these archer issues can finally be put to rest.

@bbbbbbbbba thanks a lot for your help !
@Silentine we'll be at the frontline again to tell the stories of Gaia whenever it's ready ;)

Removed isTarget(). This issue should be finally resolved.

@Silentine confirmed, we've played all day long with GoG installed, ran perfectly !
Thanks for your hard work on it. Have a great -whatever time it is at your place- !