I agree with @orioncrystalice and @eklinka that Gigas doesn’t deserve a high ranking. Here’s my more detailed grading for how I see it performing:
Strong generalist to round out a team if you’re lacking specialist counters, but that’s about it. Since it competes with other legendary options (that are way more effective) for rare candy spending, you’re better off investing into something that has more utility.
Gigas is limited to two slow quick moves and big charge moves that all cost 100 energy. Against highly motivated gyms, it’s slower and clunkier than specialist attackers. Against demotivated gyms, it’s inefficient as it will overkill most defenders with its expensive charge moves.
In PvP, Hidden Power is really underwhelming and Zen Headbutt is just straight up terrible. On top of that, Gigas’s charge moves take half of forever to fire off. In PvP, Giga Impact gets nerfed to 150 damage for 80 energy, so it’s literally just Hyper Beam with a different animation.
Team Rocket: F
Similar to PvP, Gigas is just way too slow and clunky for Grunt battles. You’re better off using Mons with better type coverage/stronger quick moves or ones that can spam charge moves frequently to stun-lock the Rocket grunt/leader you’re up against.