Orca bloodhound rush is devastating if you succeed. On smaller maps can APC rush be quite effective, (4-5 APCs with 3-4 Rockets and 1 Engineer), just plain preadator + APC spam can be effective, pitbull rush may be viable and so on. But don't get me wrong, I am by no means an expert but nevertheless attacking is key. Ask CGDIF and the like, he is a much better player than I am. Gamereplays.org also has a wide variety of GDI strategies.
But most of the time I don't try any particular strategy, I just try to expand and gain map control using GDI's heavy units and superior aircraft. Aggressive expansion if you will. By no means an expert strategy but it often works.