Here's a few tips for composing music on GBand or just in general.
1)Listen- What kind of music are you creating and what will it be for? Take some time to do in depth listening of the type of music you want to create. For example, if you are into jazz you want to go to the Blue Note to listen, not Madison Square Garden. If you creating music for video games, then pay close attention to the music on the games.
2)Imitate-What is it about certain music that appeals to you? Do you like the throbbing quarter note pulse of the bass drum in that dance track you've been listening to? The rhythmic figure that the guitar is riffing on? The slow/fast tempo of a song? With the music you like, find the things about it that you like and try to replicate them.
3)Start Simple- It is much better to have a simple product that rocks as oppossed to a sophisticated on that is luke-warm. It is much easier and effective to create a polished track that is simple. Techno+dance,rap, hip-hop are all monotonous and hypnotize with a short and redundant pattern. This is easier said than done, but the patterns are short and effective. You don't have to been extremely complicated at first.
3)Don't been afraid to go against the grain-The beautiful thing about creating is that while you may want to follow guidelines, you never have to. Do what you feel is right, even if it is against the norm. Don't be afraid to try something just because it is unusual. Bottom line, if it sounds good, it is good. For example, don't be afraid to use a banjo loop with a fast-paced drum n' bass groove or on a funky tune. Also, you don't need a MIDI keyboard , especially if this is music for a game or a promo. Find a couple of loops that work well together and add some slight variations.
That is a start. If that helps or not, let me know. I hope everything works out for you and post a re if you'd like some different advice.