120 AD for the team, for 7 seconds. That could be the whole team fight. GP's one of those "if your team sucks you're fucked" champs but he's hardly trash. He's got nice peel, good damage from q without having to build for it, extra income potential, and is a natural tank with his get out of CC free card. Sure, you'll probably only hit people for 400 damage or so with your ult, but if you dropped that on the entire team in the middle of a dive for your carries, you just hit them all with 6 seconds of a 25% slow, while boosting your allies and knocking one of the carries down to half health or lower after your damned q crits for a thousand or so.
He only needs two damage items to be a carry killer, doesn't need to be a glass cannon.
So, the bonus damage is nice, ofc, for 7 seconds. Added damage is great in a team fight ofc. Now, the damage is most useful on champs with some crit chance, so the primary beneficiaries are typically gp + the adc. More often than not people use it to engage a fight with the minor speed boost, so some time is lost, but sure - its ok an buff at max level.
He's got nice peel. But its self peel and he's still generally not going to try to be a front line with his primary damage coming from q. If he builds somewhat tanky, then that's fine, but self peel is not peeling or a stun for the team (eg less useful).
The ult is just a 25% slow.... and its so reliable that you might only get 1 hit on an enemy champ with how it works currently. 1 hit. The slow's in effect the entire time you are in the ult, but I just dont think its that strong.
I still think it just comes down to him being a lane bully, having a fairly useless ability outside of lane (orange - I mean, seriously - are you going to focus gp in a team fight with your stun if you know his orange is up? Its like stunning olaf when his ults up... kind of dumb).
Now, if my choice is between caitlyn and gp in mid, I'd go caitlyn 100% of the time. Why - because she can do more for the team than some bruiser GP build, or some full glass cannon GP, or some tank GP, or support GP (which sounds swell because of the passive boost, but its quite bad as well). Knowing what GP brings and only having a choice to play an ADC or GP mid, outside of just wanting to have fun, would anyone wisely choose GP? If so, why?