mGBA features a very basic embedded debugger. It has a help command, but this command doesn't explain what arguments any of the commands accept (or require). One has to look at mGBA's source code to get that information.
Integers are decimal by default, but can be prefixed with 0x or $ to allow hexadecimal. The 0b prefix marks binary literals. Any command which takes a memory address expects to receive it in the form of an integer.
Literals of the form $01:23 have a segment of 01 and a non-segment value of 0x00000023.