how can you get better gas milage

A few things you could do include keeping your tires at the proper psi, eliminating any unnecessary weight, using a quality full synthetic oil, keeping the engine tuned up, and upgrading your air filter.

what is the different between full synthetic oil and castrol gtx motor oil?

GTX is a conventional(mineral based) oil. This means that it doesn't perform as well as what a synthetic would. Full synthetics cost more but can generate slightly better fuel economy, reduce engine wear, and even improve power. Just make sure it is full synthetic and not a "blended" synthetic for the best results.