Rockwell can be found in the Rockwell arena beyond his designated terminal, I believe it’s beyond the Graves of the lost (or commonly called the Rock drake nests) but it’s somewhere there.
Also to fight Gamma Rockwell like any DLC boss you need the three artifacts.
How to fight Rockwell:
Firstly Rockwell is in the center of the arena, stantinary in a pool of molten element, please do note that whilst his five tentacles are up Rockwell can not take damage, he attacks by jetting out balls of energy that Beal area damage, electrifying the ground with a tentacle slam and summoning Nameless plus Elemental Repear kings.
To defeat him, you must destroy all tentacles before attacking his heart in the center, note that he can raise his tentacles again so you must repeat the proses.
Use a high level drake with lots of stam, (dont waste your time leveling up damage on your drake, it will be used for running) an ascendant shotgun with lots of shells, extra hazmat, and a buttload of Megalosaurus for actually fighting him. (The shotgun and drake are for fighting the tentacles)