I agree that he already was one. If you are looking for a cannon date you aren't going find a definitive one. Also I don't think fusion is a mandatory for being considered a crystal gem, (I.e. Peridot gas never fused). Greg has always been willing to do all he can do to protect the earth and the people that he loves, he's loved, he's lost, he's persevered, and he's fought. Yes, he may not go on missions all the time or be able to materialize weapons, but he clearly embodies what the Crystal Gems are all about.