I think you'd be better with a new pump too rather than a separate guage. Btw your tyres don't need to be run at 120psi that'll be the max, you don't want to be to far below this for avoiding pinch p'tures and it also helps with the rolling resistance but for comfort you may want to be a bit lower. With the state of our roads I usually run my tyres at 100-105psi rear and 95-100psi front and for me thats a good balance.