I'm not offended, but I know people who would be. I'm probably the least religious person in my neighborhood, being, well, an atheist.
However, I would not use such a gamertag. Yes, he has a right to do it, yes he probably doesn't mean much by using it, yes religious people can be too sensitive. Yet what does this do? What is the point aside from angering people?
I argue against religion a lot - usually to counter bad ideas in order to prevent religion from brainwashing more people. My railing against religion does serve a purpose, a good one I think.
Yet saying 'Jesus died LOL' doesn't serve any purpose. It's not an argument against religion, it doesn't send any message other than "What was an intensely emotional topic to you is nothing more than a laugh to me". This is just baiting and goading. It doesn't make atheists look good, it doesn't make anyone look good. It doesn't win atheism any friends and doesn't do anything aside from make people angry. And I fail to see the humor in that.