How do I know if a steak has gone bad?

I bought a steak months ago (it was red at the time...I would never buy discolored meat) and put it in the freezer as soon as I got home. I thawed it out slowly in cold water but now it looks off color...kinda darker and a little grey and it smells like gristle and fat. I think it may just be a poor cut. There is no sour or rotten smell, but could it still be bad? Has anyone had meat smell stong of gristle when it went bad?