That rumor was started in 2004, they started making the movie but they didn't have enough supporters and they couldn't fund the movie. From then on they tried to make it look like it was just a rumor so people don't think like Hollywood is losing money.

In my opinion, GTA would be a pretty awesome movie. Just take GTA 3's storyline and get some good actors, put the financial problems aside until the movie releases and bam, you got a hit.