Echoe is a library for quickly creating gems in Ruby. I I've hacked up a quick patch for it so you can use your .gitignore file as the manifest ignore pattern. I found it boring to re-list the ignore stuff since it in 99% of the cases (at least for me) already is present in .gitignore.
I'm sure that the patch doesn't work perfectly though. I know there is differences between the globbing syntax in gitignore and in Ruby's File#fnmatch - the latter is stricter. But it still works perfectly for the (small) projects I've tried it on.
Usage goes down like this:
Echoe.new(PROJECT) do |p|
p.ignore_pattern = FileList[".gitignore"]
Grab it from my fork on GitHub.
UPDATE: This feature is now included in echoe 3.0.2!