GP used to be kind of a 1 trick pony building lots of Crit and hoping for a big gunshot every 4 seconds or so.
After his rework he is much more suited to playing more conventially like a tanky DPS, which is good. Critplank literally just walks around, and every 4 sec he runs in, gets off 1 shot and runs back. Basically playing like a really bad AP carry with 1 spell and bad range.
I would recommend the Tanky DPS approach and check guides on Solomid on how to do so. I like Triforce for offense since it adds 160ish damage to every single gunshot you use reliably just from the proc. (plus the AD and crit boost). The slow proc is nice to have and triforce also adds the big movespeed, a bit of survival, mana, and AS for melee.
Besides triforce you will want tankiness for the rest of the build, including CDR for faster gunshots and more ULT availability.