Will Gunn have a say in the Guardians participation in the Avengers? He should.
He will.
But honestly, after this I kinda feel like I'd prefer The Guardians/Thor/Nebula versus Thanos rather than The Avengers as a whole. I mean these Guardians have taken down one guy powered by an Infinity Stone AND a fucking Celestial being. I'm sure Thanos will be tough to kill, but I don't really see what Cap et al bring to the table particularly.
It's the same problem I had with Doctor Strange joining The Avengers. Strange/GOTG/Thor go up against cosmic/magical beings... they're so far removed from The Avengers as you can get, and I'm worried they'll all be nerfed so that Cap/Stark get their big moments.
There's also the fact that The Avengers only have 'protect Earth' as their stakes. Thor has 'you tortured my brother', Gamora and Nebula have the shit that Thanos did to them as kids, and Drax has the death of his family to avenge. So it's going to be odd if none of them are the ones to take Thanos down - while, at the same time, from an MCU focus it'll be weird if Iron Man doesn't get his big moment (RDJ is still 'the guy' in the MCU.)
So it's going to create a weird film because everyone has to have their moment, but Gamora or Nebula or Drax not killing Thanos is going to feel like a bit of a letdown.
And that's Gunn's fault I think. Well, fault is the wrong term. But he's done such a great job with that team and those characters that it's skewed things a bit away from it being an Avengers crossover. It feels kinda like how Fear Itself would've worked better as a Cap/Thor mini-crossover than a line-wide event.