A 205/55 seems to be an incredibly rare size, hence too expensive to be worth it. Likewise, I dont think its a "safe size" so you might want to find if some places will even sell you a size like that.
on another note, I dont think the 15" GT wheel will support a 205, stock for the GT was a 185/60 HR15 Eagle GT+4, the one 195/60 was a replacement (Eagle RSA) used because the 185/60 was discontinued
If you want width in your rubber consider going to a 16x7 or a 17x7 rim. On my 17´s I´m running a Kumho 215/40 YR17....and the best part of upgrading to a 205 or a 215/40 is due to trend they´re cheap. You can a Nitto 205/40 tire for under 100 bucks now.