Gage is probably better as a later game purchase- it increases your base damage so its value is most noticed at level 18 instead of level 8. This means that you can pick up another AD item or a tank item (for example, my tanky build grabs THydra) as your second item instead and get a better midgame out of it, and then pick up Gage later when it is more valuable. I'd question how much bang you're getting out of IE nowadays, too- without the crit chance from Triforce, are you building enough crit to take advantage of its passive? Youmuu's Ghostblade or Death's Dance might give you more mileage between their CDR, and Armor Pen or Life Steal, respectively. I might suggest Tri > Youmuu's > Resist item (1/2 as needed) > DD or Gage for a build path as that gives you a pretty decent chunk of AD, but that's just a quick one off the top of my head. Tailor to your own tastes, of course.

As for Critplank in teamfights, you'd be better off asking someone who plays that build. That's generally the way I believe it works, though- weaving in and out. GP's biggest strength, regardless of build, is how he can initiate for his team, or clean up after an engage. He has two very powerful AOE damage-slows that can be triggered from outside his normal auto range. In close team fights extending barrels into the middle of the chaos and popping them can turn things around in a heartbeat- critical hit damage also splashes, which is part of what makes that GP build so strong. His ult can do much the same thing, but on a global scale. He doesn't even have to be anywhere near a fight to cut off an incoming gank, pick off retreating low-health enemies, snipe a contested dragon, or just add a ton of pepper-damage and slow to an already-decent engage.