The abort command is inherently asynchronous (potentially overlapping with another command), whereas the regular GTP command stream is synchronous. So it is not very convenient to see abort as a regular command.
It seems to me that the fact that the gtp stream allows command id numbers which are mirrored by the response is a feature that implies the possibility of asyncronous response.
The gmp seems to lack a few things such as a mechanism for passing komi or resign moves. Moreover the document defining the protocol is quite old. Since the gmp is still the standard for tournaments this circumstance seems unfortunate.
Dan