A GPL-licensed project can be forked. But, if you release work that uses or is derived from the GPL-licensed project, among other things:
1. you must include or make easily available the source code for your derived work.
2. roughly speaking, the license of your derived work cannot be less free than the GPL license (i.e. cannot infringe on rights granted by the original GPL license).
Releasing work derived from a BSD-licensed project does not have these requirements.
References: https://en.wikipedia.org/wiki/GNU_General_Public_License#Ter...
(edit: and on second glance, sorry, it seems like these details aren't really that relevant to the point of your questions.)