Is tying up a guy and tickling him gay if you're a guy too?

It depends. Realistically speaking, actions are not gay. If you want to tie up another male and tickle him because it gives you sexual gratification, then perhaps you should think more on your sexuality. "Gay" is your sexuality, not something that a single action defines you as. Think about your purpose or thought behind doing this - why did/will you tie up a guy and tickle him? If your friends and it was/is simply fun, then that's awesome that you're comfortable like that with one another. If this was a past action that you are concerned may suddenly define you as gay/straight or cause people to judge you, then don't worry. A single action doesn't make you gay/straight - it is whatever you define yourself as.