Gigrin is a weird place ,the action is extremely fast ,so in reality you have to be at one with your equipment ,you will certainly get some photos with the 70-200 although a t.c would help with reach I feel it would slow down a/f a bit to much ,and as you have to pay a tidy sum for the best hides it's best to be ready for it .my recommendation would be to go once on a trial basis and see how you get on then appraise the gear you will need for a further visit .
I would suggest a minimum of 400 MM. on a fast focus lens coupled with a decent tripod and gimble head as rest assured your arms get tired quickly .think of yourself as akin to a rear gunner in a WW2 bomber and there's planes coming at you from all directions .i know when I go the action lasts for about hour and a Half to two hours and that's virtual continuos shooting with a shutter count getting to around 2000 shots
You also need to be familiar with the exposure compensation on the camera as one minute they are in the sky next down at ground level. Then back up again I always find a practise with local gulls is pretty good practise for Gigrin and well worth doing