You can use the assets found in the AppGameKit product for your own games providing you credit 'The Game Creators' for the copyright on those works included. Also bear in mind some of the games are already published in various app stores so simply re-compiling them and selling them as your own games would create confusion in the market. Another side effect is that end users might recognise the art as from AGK/TGC, lowering the uniqueness of your title. A good 2D artist is worth his weight in gold, so I encourage you to find one as soon as possible, make him your best friend and give him lots of gold!
I drink tea, and in my spare time I write software.