IIRC all the reports were that Motte was holding out for a two-year deal which is why he didn't sign until the middle of September. Obviously he didn't get it, but the Rangers weren't going to wait around.
As far as Gauthier, I do think he's a decent bottom six option, but we don't really have much space for a RW who doesn't score much and doesn't kill penalties. He cleared waivers earlier this year and it's not like he has suddenly become a dramatically different player since then. He is what he is. If he ends up becoming that guy who blossoms at 26, good for him.
Seems like a reasonable move, trading a guy with very limited value and who likely wasn't in your plans for a guy who fills needs on your current roster.