what can BASIC do that GM can't?
GM script language omitted a few BASIC features, but it also added GM-specific things like dialogs and prompts.
Notably, GM script language does not allow arithmetic expressions involving more than two terms or factors, so e.g. these two lines
will work, but these won't:
X = 1 + 2 + 3
X = 1 * 2 * 3
Furthermore, GM scripting does not support parentheses.