Where do you find Gible in the Wayward Cave?
It says its in a "secret area"... Where is this?
There is a hidden section of Wayward Cave. The entrance to this secret part is directly under the Cycling Road, so you can't actually see the entrance. Just go north towards the normal Wayward Cave then go under the cycle road and use trial and error until you enter the secret entrance.
How much attack does Infernape need to guarantee an OHKO on Blissey with max IV/EV with Close Combat?
305 Attack = 85% - 100% Damage
359 Attack = 100% Damage
Infernape can't hit 359 attack so it cannot guarantee a OHKO on Max HP/Def Blissey without Choice Band/Expert Belt/Life Orb.
With Life Orb Infernape only needs 270 Attack to guarantee a OHKO on Max/Max Blissey. 229 is the lowest possible attack stat that can OHKO Blissey with a stabbed Life Orb Close Combat (85% - 100% Damage).
What would be the best set for my Adamant Gyarados with very low speed, high hp and high attack?
You
could still use it as a bulky DDGyara. However you could try a Choice Band or even a Sleep Talk Gyarados.
CB - EQ / Waterfall / Ice Fang / Stone Edge
Stalk - Waterfall / Rest / Sleep Talk / Stone Edge or EQ
I'd personally use the CB set to hit harder then usual as if it is really slow I doubt you'll even get close to outspeeding anything that you need to, i.e. Gengar