Important Note:
Gaffer tape isn't generally waterproof. I have a lot of professional experience with gaffer's tape. If you're in a climate controlled photo studio or sound studio, it's great... if you buy the high quality stuff. It's easy to tear, doesn't leave a residue etc. I would never count on it outside or in a damp, sunny, wet environment though. Most gaffer tape loses it's stick when wet.

There are some gaffer's tapes that I've seen listed as waterproof. These I believe are the exception and not the rule, however. I've never used these.