I'm not sure, this is not an easy topic.
GPL is a "viral" license, which asks you to redistribute your work under that license, if you mix gpl content with your own strictly enough (e.g. running in the same process?). GPL is a license written for code, and AFAIK it does not clearly explain how to use assets under that license. I have not understood if you can use GPL assets in a NON-GPL program... or mix GPL and NON-GPL assets in the same program. Just to be sure, OA uses GPL for everything.
GPL allows you to sell your software (giving that your customers are allowed to easily get the "sources", without the need to pay a second time), so in theory you may make everything GPL and still sell it, BUT probably the other assets -those you bought- do not include "sources" or a GPL-compatible license (futhermore, I don't know if you can somehow prevent third party from using your GPL program without paying you, after the first copies start appearing online... I'm not a laywer.)
About asking the authors of those two models to authorize you to include those assets in your game, or otherwise to release them under CC0 or Public Domain, that may be a possible way... IF their authors are okay with it.
I'm not sure who are their authors. Arachna is
probably a model by
Fromhell (the project leader, she is who does everything about OA models nowadays, maybe one of the few persons in the world still capable of creating models in MD3 format).
The wiki page about
DO NOT LINK[/b]) h t t p s : / / openarena . wikia . com/wiki/Sergei]Sergei mentions "Modeled, animated and skinned by enki. Voiced by Rottking."
There is also
an additional skin for Arachna, called "Forlona", which is not included in OA 0.8.8 (but you can see it in the Android port), which is made by
Wayuki.