The assembly code was originally written for the "GST 68000 Macro Assembler A246V040". I still found a mention in a review of ST assemblers but otherwise the company seems to have disappeared and no documentation is to be found anywhere on the Internet.
Dev-Docs.org has https://docs.dev-docs.org/htm/search.php?find=gst which is GST’s C compiler, which includes their assembler:
Dev-Docs.org has https://docs.dev-docs.org/htm/search.php?find=gst which is GST’s C compiler, which includes their assembler: