Welcome Kymco Owners!
the original rollers are 20/ 12 15 1/2 gr. the most popular dr. pulley sliders are 20 /12 14 gr. I bought 14 and 15 gr. slider at the same time as I did not know which I would end up with,.
PS Downtown and GT.300i are the same engine and tranny you can check by looking up the parts on line..