Gambas is like VB6 but on the Linux desktop. For a simple tool, it’s easily my go-to.
Linux GUI programming can be a bit of a pain. There are tons of frameworks and not very many expanded tutorials or books, at least compared to most GUIs.
If I had to write a GUI today, I’d pick something like Flutter so that I get the benefits of mobile app developer tools (that are generally better IMO). I’m looking into Slint in my free time, but I don’t think that’s something that’d fit you.