Programmatic GDB completer interface via Python

Yes, there is a Python API. It is documented here under the function Command.complete (text, word)

The default completer will do filename and sub-command completion but you can extend that by providing your own custom complete method. Here is an example:

class MyGdbCommand(gdb.Command):
    def __init__(self):
        super().__init__("mycmd", gdb.COMMAND_USER) # or whatever command class you deem appropriate

    def complete(self, arguments_string, last):
        is_brk_char = (len(arguments_string) < len(last))
        args = gdb.string_to_argv(arguments_string)
        if arguments_string == "" or arguments_string[-1:] == " ":
            args.append("") # Add dummy argument to complete
        argc = len(args)

        if argc == 1:
            if is_brk_char:
                return gdb.COMPLETE_NONE
            if args[0] in ['-l', '-o']:
                return args[0] # it's complete
            return ['-l', '-o'] # valid option flags

        if argc == 2 and (args[0] == '-l' or args[0] == '-o'):
            if is_brk_char:
                return gdb.COMPLETE_FILENAME # -l and -o take file arguments
            (head, tail) = os.path.split(curr)
            return getMatchingFiles(head, tail) # implement appropriate heuristic; pass args[0] flag if it matters

        if argc == 3:
            if is_brk_char:
                return gdb.COMPLETE_NONE
            if args[0] == '-l':
                return ['-o']
            if args[0] == '-o':
                return ['-l']
            return []

        if argc == 4:
            if is_brk_char:
                return gdb.COMPLETE_FILENAME
            (head, tail) = os.path.split(curr)
            return getMatchingFiles(head, tail)           

        if is_brk_char:
            return gdb.COMPLETE_NONE
        return [] # No more valid options

NOTE!! The completer is not responsible for validation. That should be done once your command's invoke() method is called.