Permissions are a pest... basically, you need to make sure that all of those developers can write to everything in the git repo. If you put all the developers in a specially-created group, you can, in principle, just do:
chgrp -R <whatever group> gitrepo
chmod -R g+swX gitrepo
Then change the umask for the users to 002, so that new files get created with group-writable permissions. The problems with this problem are legion; if you're on a distro that assumes a umask of 022 (such as having a common 'users' group that everyone is in by default), this can open up security problems elsewhere. There's also the problem that sooner or later something is going to stuff up your carefully crafted permissions scheme and then the repo is out of action until you get root again and fix it up (rerunning the above commands, basically).
A superior solution, though one which is less well understood (and which requires a bit more OS/tool support), is to use POSIX extended attributes. I've only come to this area fairly recently, so my knowledge here isn't as hot as it could be, but basically what an extended ACL is is the ability to set your r/w/x attributes on more than just the default "user/group/other" sets. So, again, you create your group, then run:
setfacl -R -m g:<whatever group>:rwX gitrepo
find gitrepo -type d | xargs setfacl -R -m d:g:<whatever group>:rwX
This sets up the extended ACL for the group so that the group members can read/write/access whatever files are already there (the first line), then also tell all existing directories that new files should have that same ACL applied (the second line).
Hope that gets you on your way.