GN now lives at https://gn.googlesource.com/.
To roll the latest version of GN into Chromium:
python //tools/gn/roll_gn.py; this will modify //DEPS and //buildtools/DEPS to point to the most recent revision of GN and create a commit with a list of the changes included in the roll.
git-cl uploadto upload the commit as a change for review.
If you don't want to roll to the latest version, you can specify the SHA-1 to roll to use as an argument to the script, and it will be used instead.