Eagle737 - I think you answered your own question.
"Retract the landing gear after a positive rate of climb is indicated on the altimeter."
That means the altimeter.
If Boeing had wanted you to measure positive rate of climb on another instrument, they would have stipulated that - or omitted the altimeter from the instruction.
Verifying it off all the other instruments is a good idea... but I can't help wonder why so many have a problem with just calling it off the altimeter as stipulated.