Is the source repository for the project at http://code.google.com/p/findbugs/source/checkout ?

Does this include findbugs itself as well as the eclipse and other related software ?  It looks like it does.

If there any reason a more modern build tool such as maven or gradle is not used to build the project ?

I see a number of things which modern Java projects don\'t really do anymore :
* IDE specific files checked into SCM
* JARs of dependency projects checked into SCM
* Manual construction of OSGi descriptors

This is just from a short peak at the project.

Is the reason a political matter (committer(s) have some negative bias against moving on from Ant) or simply that no one has attempted or the time to convert the project.  Obviously there maybe a training aspect if the day-to-day operational maintenance is affected by such a disruptive change.

There is also the aspect of using SVN versus GIT would that be seen to help having more heads working with and contributing to the project.