You do not need a gamebit set.

It is different from version to version. It also depends as to whether or not you want to just get the cover off, or totally disassemble the PS2.

It's far too involved for me to type it all. Pop off the black square plastic caps. Do a Google search. Remember where the long screws went and the short screws, and pull the cover off slow and gentle, watching for the ribbon cable. You tear that free, and unless you are VERY handy, or very lucky, it's SONY refurb time.

Good luck. I am very good with PS2's (repaired more than 50), and I know enough that you need to take great care. Half of those 50 were further broken by the owners attempts at disassembly. They are like a laptop inside.