Thanks a lot! I'm glad you enjoyed it!
I don't use GB Studio, my gameboy & gameboy color ROMS are made using GBDK, a programming library for the C language. I highly recommend it if you're a little familiar with code. GBDK does have some issues, as it is an old library, and can cause memory leaks even with some otherwise-normal code. I would highly suggest using GBDK-N or GBDK 2020 instead, which are both very recent updates to the library that attempt to fix its issues.
For this challenge, I've also been using gingemonster's GameboyPngConverter to help with the tile generation. (Before that, I'd go through the usual GBTD and GBMB tools, which are ok for regular level building, but aren't thought out for pixel art pieces like this)