Can the new wads found in the newest release be used in GZDoom? I see some wads I haven't seen before, such as id1.wad, id1-res.wad, and so on.

The one's intended to be played (e.g. sigil.wad, plutonia.wad, etc.) seem to load just fine in gzdoom. id1.wad is the new official expansion "Legacy of Rust". For the rest, you can get an explanation on this Steam-thread:

https://steamcommunity.com/app/2280/discussions/0/4511003584755540256/#c4511003584755690614

tl;dr: Resource packs for modders to use.

Huh, well that's a lot easier than I was fearing. Awesome, it works great in the latest GZdoom.

id1.wad is the new episode, Legacy of Rust. It works fine in GZDoom (use with Doom II as the IWAD), though there's a couple of bugs in GZDoom's implementation of MBF21 features so you should use a development version from DRD Team.

id1-res.wad and id1-weap.wad are the Legacy of Rust resources for modders. You don't need them to play.

The other new wad is iddm1.wad. It's a deathmatch pack.

I'm guessing masterlevels.wad can also work with DOOM II as the IWAD?

Master Levels is just an addon for DOOM II. So, yes. It requires exactly doom2.wad as an IWAD.