I have kept giant Pacific octopuses as a course of my research. First of all, having kept them, I had a 750 gallon tank. That tank was a bit small and had I kept the octopuses for more than two months each I would have wanted to put them in a larger tank. Secondly, you will need to get a collection permit from the state or province from which you are collecting them. In Washington state, where I have collected, they are touchy even for scientific collection, so it will likely even be more interesting to convincing them to let you collect one as a pet. GPOs are "game fish" in most of their range with makes governmental organizations less likely to let people capture them to keep as pets.
A well fed GPO will easily be an 8-foot, 60lb animal before it dies. Keep that in mind.