In talking about it Phelps said he and Gary agreed to disagree.
I'd all but guarantee that Gary would rather not have diabetes than have it and gain some sort of advantage from the insulin shots. Keep in mind that he was diagnosed between the 00 Olympics and the 04 - it's not like he was an average swimmer than suddenly became amazing b/c he started on insulin. He was darn good before his diagnosis and more impressively darn good after.
All that said it is pretty sad that a 19 year old was more mature than he was ... he whined that Michael didn't earn his relay spot, well neither did he.