Lots of editing software out there for phones, I use Prisma occasionally, though not for GT pics. I did a quick search on the computer and came up with this one. Played around with it a bit, has a lot of options. Looks like it is pay to use, but I just did a screenshot, and dropped it into paint (yeah I'm old school) to save it. That said, don't discount the photomode effects in the game. A lot can be done with them. If I had one piece of advise, learn how to adjust your black levels. Well, that and I recommend using the same filter for both the car and background, making them different tends to make the pics look very fake.
Transform your photo into a cartoon with a single click! Select from several cartoon effects and filters that'll turn your image into a work of art in seconds!