Evan Gattis has a unique back story, which you can read here. He really brings one key asset to the team, and that is power. In his first two years in the majors, he has totaled 43 home runs, with a .487 slugging percentage and a .234 ISO. He played catcher the last two years, but the Astros obviously don't need anymore of those, so I expect he'll play LF/1B/DH. He certainly won't help the outfield defense, but the hope is he will hit well enough to make up for it. He certainly did in the first half of last year, putting up a .290/.342/.558 line, good for a 149 wRC+. He stumbled in the second half, but that could have been the result of some back troubles, which is hopefully behind him. Although he is the most established name in the deal, he is still under club control until 2019.
The Astros gave up...a lot. Mike Foltynewicz, Rio Ruiz and Andrew Thurman. I get the rational behind each, though. For as fast as Foltynewicz throws the ball, he has proven very hittable, with a 5.08 ERA in AAA and a 5.30 ERA as a reliever in the majors. The hope is that he can improve his secondary stuff and command, and become either a middle rotation starter or a shutdown reliever, but right now he is mostly just dreams. Rio Ruiz could end up being the biggest piece, as had a good showing at Hi-A last year. It's clear the Astros had him behind Moran, and he was most likely three years away. Still, if he lives up to his full potential, it could hurt. Andrew Thurman has been a major disappointment, but is still young enough to turn his career around.
If the Astros can find a position for Gattis and he puts up big offensive numbers in a park that seems perfect for him, this deal will be a positive for the Astros. They traded from areas of depth, and got back a major league asset. This, more that anything that has happened before it, signals that the Astros are entering a new phase of development. We will see whether Luhnow can pull of this stage over the next few years.