On Retina Macbooks (and now I guess Retina iMacs), gitk works in low resolution mode. Considering the aesthetics of gitk this is probably one of the smaller problems, but I found it a tad annoying and decided to scratch something together to make enabling it a bit easier.
curl -s https://gist.githubusercontent.com/cynthia/5f2355a87c2f15d96dbe/raw/6727e73a007b0efabf55dd065e588467ffccc016/wish_app_info_plist.patch | sudo patch /System/Library/Frameworks/Tk.framework/Versions/Current/Resources/Wish.app/Contents/Info.plist; sudo touch /System/Library/Frameworks/Tk.framework/Versions/Current/Resources/Wish.app
This one-liner will make it run in Retina resolution. The actual patch is fairly trivial, but for those who want to double check if there is anything malicious it's available here.
The warning "patch unexpectedly ends in middle of line" can be safely ignored.