I tried to change my Gamertag to MusclesGlasses (Because I'm a huge Epic Meal Time fan) and it appears the Gamertag is Invalid. I tried to enter it in my Profile on my Xbox and it said the account was in use but when I tried searching for it using the "Find A Friend" search box under the "Friends" category on Xbox.com, it said that the Gamertag was invalid.
I added a space between the words and a profile popped up (lucky guy) but when I removed the space, the Gamertag was said to be invalid again.
My thoughts are that the last 5 letters of the word "Glasses" might be picked up by a Live censor which is used to prevent people from making racial or explitory Gamertags.
I think this because my friend tried to make the Gamertag "Spicyness", but he said he could not because the censor said his Gamertag contained a racial slur. He reluctantly made "Spycyness" instead.
I talked to a support representative about this on the phone and he suggested changing some letters to numbers and seeing if that worked but I prefer to have just letters and keep it original.
Any information is greatly appreciated and keep working hard.