Matthews isn't the competition for Gibson. I mean, they're probably in competition for the last spot but not directly with each other. The guy who's making the team between them really comes down to Landry, imo.
Why? Because Gibson and Landry are in many ways mirror images of each other, and play similar positions. They're both relatively slow, relatively unathletic guys who are smart, catch the ball reliably and play better than they time. If Landy shows well in training camp, like he can start inside right away, then Gibson will be gone. No need to carry Gibson's cap number.
Matthews is a more direct competition for Hartline... since they're both at their best outside the hashes. Hartline could probably play inside if he wanted to (he's not the ideal physical specimen to do it, but he's smart enough), but Matthews just doesn't have the savvy yet to play inside full time. Therefore he's not a direct threat to Landry or Gibson for a starting position (we play without a fullback, so the third WR is sort of a defacto starter). If the FO and coaching staff considered Matthews a viable inside option, they wouldn't have drafted Landry.
If Landry doesn't play well in training camp and in preseason then that might force the team's hand relative to Matthews, since Gibson will have to take up that role at least to start the season. You can't really be without that guy on this team because Tannehill loves throwing the ball inside the hashes. He's been that way since Ryan Swope was his favorite receiver at aTm.
Fwiw I think Landry will show well enough so that Gibson doesn't make it and therefore Matthews does. The tie often goes to the young guy with a cheaper contract in these situations, as it should. Not that I'm as high on Landry as some around here. But I think he'll show enough.