G&W vs Link is +2G&W. Unless Link plays extremely smart G&W will find opportunities to punish him. Get Link offstage at any % and you have a good chance of easily gimping him.
Link can live forever but G&W's great intercepting and recovery shuts that down. Link's ground game beats G&W because Link's reach but G&W jumps circles around Link's air mobility. Link has less power but G&W's lighter.
Link's advantages are: better reach, better spamming that can't be bucketed, bombs/Zair beat every attack G&W has (except the invulnerable part of UpB and Uair from below), Faster KO moves, weight, versatility.
G&W's pros: Faster attacks in general, MUCH more mobile and fast in the air, devastating offstage game, great punishing options, good damage racker, harder to punish unless G&W is being stupid, power, can spam attacks (Bair/Nair/Dtilt etc), attacks eat arrows and boomerang, no easy-killed situations and does well anywhere Link does well and can CP stages for a bigger advantage.