You cannot fix a too-tight saddle with extra pads. It's like putting thick socks on with shoes that are too tight.
Some pictures of the horse and saddle would help. Take them with saddle on, no pad, lightly girthed. Get a side shot (standing on level/hard ground with head up), a back view (stand on something if you need to), and a 3/4 front view of his WHOLE shoulder (a close up of the withers tells us nothing).
As for gel pads, most are worthless. They will trap and even create heat and the gel simply moves away from pressure, so there is no shock absorption or comfort for the horse with a saddle that doesn't fit just perfectly.