GTD is popular among geeks, but how people adopt it seems to vary slightly. One version is Charilaos Skiadas’s GTDAlt bundle which he has documented extensively on his blog.
I should add that to install a third party bundle: just double click it! It will then be installed in
~/Library/Application Support/TextMate/Pristine Copy/Bundles.
Should you modify the bundle, your changes are stored as the differences in
~/Library/Application Support/TextMate/Bundles and it is possible to then later get a new version of the third party bundle, double click that one, and then only have the parts in
Pristine Copy overwritten by the update.