I'm skeptical, but what about it besides the timing makes it as obvious as April Fools jokes can get? It was posted on the 30th of March by the way. There is nothing too outlandish in the description like you would normally see with an AF joke, and even though the cover looks photoshopped people have felt the same way about other games and a few of the HD collection games that have been released, namely GOW Origins Collections. Even if it's fake Sony is stupid to make it happen and take our money.
It makes no sense for a start. HD collections are all series, games you play one after the other with stories. Three GT games in one collection makes no sense. Who would want GT PSP on the PS3 when you have GT5, basically the same thing on a larger scale? Not to mention Scaff saying a few days ago that the games would never get re-released on the PS Store because they would need to re-acquire all of the licenses as it's a new release. Same deal here.
Also:
One updated Gran Turismo 5 will be available for Christmas 2012 in order to allow the possessors of GT5 to be able to confront the players of Gran Turismo HD Collection on special circuits in common with their own favorite stables!
The translation isn't great of course but what it's basically saying is GT5 players can take on GT4 and GT3 players at the same time. How? The games all have different physics engines. Not possible.
Oh and finally, these are games made by PD. If they went to convert three games to HD it would be released in 2064.