Donkey.bas was most likely written as an educational exercise, although IBM ordered it. Microseft needed to show people how to code with their interpreters and compulers too, so Donkey.bas is not that sophisticated.
Gates was responsible for writing the first Altair Microsoft BASIC interpreter, except the bootloader, the software for the Kyotronic clones (designed in collaboration with Kay Nishi and licenced to Kyocera, Tandy and others).
Gates also had ample experience in programming minicomputers such as DEC PDP series together with Paul Allen.
You can read all about it in all the Bill Gates biographies out there.
Gates was responsible for writing the first Altair Microsoft BASIC interpreter, except the bootloader, the software for the Kyotronic clones (designed in collaboration with Kay Nishi and licenced to Kyocera, Tandy and others).
Gates also had ample experience in programming minicomputers such as DEC PDP series together with Paul Allen.
You can read all about it in all the Bill Gates biographies out there.