step 1: get the console there's already a bazillion guides for that already so i wont tell you.
steap 2: sv_cheats 1 basic command for enabling cheats or commands
steaeap 3: enter command physcannon_mega_enabled 1 (or copy and paste this one)
steaeaeap 4 (optional): once your done with the super gravity cannon fun, just type into the console, physcannon_mega_enabled 0 and it will go away
steaeaeaeap 5 (optional): bind the commands to keys like with the command "bind h physcannon_mega_enabled 1" and/or like the command "bind j physcannon_mega_enabled 0" (i put the commands in quotes so they are less confusing to have multiple of)