Since Tremulous was released in 2006, many mappers had been done for original source engine like using GTKraidant, including ATCS, Nexus 6, Niveus, Uncreation and much more, also some Trem users had made so much mapping like UTCS, Nano! etc., they actually made with NetRadiant for compatibility support, as the fork of GTKradiant.
But these are pretty old and outdated version of render engines. Afaik NetRadiant are the latest version of ioquake3 engine renderer, but it’s been few years as Ignar released it in summer 2015.
Until one day, a name Garux appeared in Xonotic forums had released a new version called NetRadiant-custom, fork of original NetRadiant, he made much improvements from rewrite old version for optimized performance. But unfortunately it doesn’t supported for Tremulous as he want to heavily main focus on Quake, Quake3 and Quake Live.
DON’T WORRY!!! NetRadiant doesn’t support Tremulous WASN’T mean fully unsupported!!!
It’s just didn’t added Tremulous gamepack for compatible lists. So you need to download this gamepack to patch manually into it and will fully compatible with Tremulous engine.
NetRadiant-custom in GitHub: https://github.com/Garux/netradiant-custom
Download in releases: https://github.com/Garux/netradiant-custom/releases