The reason ycm and vim-gitgutter don't work well together isn't the "plugin interface"; it's the limited nature of the sign column which can only display one sign per line. If both ycm and gitgutter want to display a sign on the same line, only one can succeed.

Edit: oops, ycm doesn't use signs...I was thinking of syntastic. As far as I know ycm and vim-gitgutter should work together.