About a year ago I posted some solution in this forum. But it needs some work in Terminal.app.
G5, and now A6 do not run on a case-sensitive disk per se. All new Macs have their disks formatted now case-sensitive, journaled hfs+.
The solution is to create an appropriate .dmg file, mount that and install G5 resp. A6 on that.
The following commands did work for me.
hdiutil create -megabytes 70 -fs HFS+ -volname g5 g5.dmg
sudo hdiutil mount -mountroot /mnt g5.dmg
Then installed Geneforge 5 on g5. That did it.
For playing afterwards you just have to mount the .dmg file in Terminal.app. The mounted volume will show up in the finder.
sudo hdiutil mount -mountroot /mnt g5.dmg
Good luck.
Edit: for Avernum 6 you will need larger .dmg file, something like that:
hdiutil create -megabytes 120 -fs HFS+ -volname a6 a6.dmg