There is a bit of GTA merchandise on Rockstar Warehouse:
GTA IV/EFLC T-shirts:https://www.rockstarwarehouse.com/Category....es&c=GTAApparel
Also keep on checking eBay for GTA stuff.
The reason why you don't see so much GTA merchandise is because it gets bought so fast. For example, a few weeks back a key that gamers received after completing GTA IV 100% when the game was first released went on sale on eBay, and it eventually sold for approx. $200.
Rockstar have released tons of merchandise for their games (extremely limited edition items like a Statue Of Happiness statue, license plates, various GTA stickers, and so on) and they all get snatched up by fans faster than you can say "Grand Theft Auto merchandise".