It's been covered, and is probably in your PS3 manual, or sonys website.
-Put a USB flash drive into your PS3.
-Scroll up on the game menu of the XMB until you get to "save data folder"
-Go into the folder and select the game in question, in this case GT5
-Press triangle, then select "Copy"
-Choose your flash drive, it will likely be the default selection
-Press X, wait for it to copy, it should take about a second.
To recover the back up:
-select your flash drive via the menu
-select the save data
-Triangle, copy to PS3
