Buddy tremulous is opensource, everything is fair use you really don't have to give anyone credit it's out of courtesy. Do as you please and give credit or don't it's not a serious issue.
Wrong.
The Tremulous source code is distributed as GPL v2. GPL v2 does not require attributing the original author.
The Tremulous (1.1) game data is distrubuted as CC-by-sa-2.5, which DOES require attribution.
Third-party content (e.g. a HUD, a map, ...) can be distributed under whatever license the author
chooses. Some require attribution, others do not, and some prohibit changes and redistribution.
If you want to use someone else's work, the best way is to ask the original author.
In practice, when I use someone else's work, I always try to at least give credit
in the README (and check the original license).
I have found that most authors are more than willing to share their work if you ask them nicely.
@HermitCrap:
Your HUD is probably not a problem, Just add a README.txt file that says what stuff you used.