A version of Cartesian Genetic Programming can be downloaded from the website. Push-gp can also be downloaded.
First you need to decide which encoding scheme you need, a tree, registers, directed graphs, grammatical rules. Then you can try coding them or use again a package of your choice.
You will need to define a good fitness function too. Without it, any form of GP will not work.
I would read a field trip to Genetic Programming from from W. Langdon.
Please go through A beautiful text on this topic by David E Goldberg, an american computer scientist, civil engineer & professor at university of Illinois. Th title of book is Genetic Algorithm: Search ,Optimization & Machine Learning. it is one of the most cited book in computer science.