I am running a
git gc --aggressive
git is supposed to be always safe from interruptions like this. If you are worried, though, I suggest Ctrl+Z and then run a
git fsck --full to make sure the system is consistent.
There are a number of git-config variables which might help your git-gc go faster. I use the following on one particular large repo, but there are many more options to randomly try (or carefully study, whichever).
git config pack.threads 1 git config pack.deltaCacheSize 1 git config core.packedGitWindowSize 16m git config core.packedGitLimit 128m git config pack.windowMemory 512m
These only help if your problem is that you are running out of memory.