I also play GW2, on a nearly 5 years old GPU and have zero problems with FPS drops when running Discord on my other monitor.
Also 30FPS musst be a stretch then, or your settings are super low/minimum because GW2 is a quite hardware hungry game. Especially CPU wise (single core only). I don't get how someone can enjoy a game that way. Especially with that semi-fast and interactive combat system that GW2 offers (dodge system based).
Not even that much. Here is a ~700 € 60 FPS/1080p build. https://de.pcpartpicker.com/list/kQY2sZ
Maybe exchange the processor for one from intel with more single core power, since MMOs (and GW2 especially) are very single core CPU heavy and you should be fine.