Pro Salamence:
Dragon typing provides resistance to Grass (making it X4 resist versus neutral on Gyarados) and Electric (making it neutral versus X4 on Gyarados).
95/80/80 defenses, making him better against Physical attacks (slightly) than Gyara
100 base Speed is significantly better than Gyara's 81 Base Speed, allowing him to outspeed many threats while not DD'ed up.
110 Special Attack versus 60 Special Attack makes Salamence much better at having a Mixed Set to deal with Physical Walls
Pro Gyarados:
Water typing provides resistance to Ice (making it neutral versus X4 on Salamence) and Steel. These can be considered to be better than Salamence's sets of resistances because of Ice Shard and Bullet Punch.
Not weak to Dragon type attacks
95/71/100 defenses, making him better against Special attacks (significantly) than Salamence
Taunt access allows Gyarados to stop attempts to Roar/Status/Haze him.