GBVM
GB Studio's game engine runs on a stack based virtual machine called GBVM game engine. You can access the virtual machine directly by using a GBVM Script event in you game.
GBVM Script
Script
VM_PUSH_CONST 0 ; Y coord
VM_PUSH_CONST 0 ; X coord
VM_PUSH_CONST 128
VM_LOAD_TILESET .ARG0, ___bank_bg_cave, _bg_cave
VM_OVERLAY_SET_MAP .ARG0, .ARG1, .ARG2, ___bank_bg_cave, _bg_cave
VM_POP 3
References
Learning GBVM
If you want to know more about GBVM and how to use it then check out GB Studio Central's getting started guide, Learning GBVM.