Yes. The GPL explicitly says this about "further restrictions":
> If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term.
However, this doesn't really even come into play because the citation request is not a restriction on the license. It's not anything. As far as the GPL is concerned, it's just some code, and the GPL grants you the right to redistribute modified copies.
And by renaming it to "free-parallel" you have respected the author's trademark. You can absolutely do this, at the cost of the author being upset at you. They might get upset that "free-parallel" is too close to their "GNU Parallel" trademark but I (IANAL) don't think they'd be legally right about that. GNU Parallel coexists with other software called "parallel".
