The real problem is that GDI needs really good balance between offense and defense on this map.
You need two tanks in base, blocking the sides of the AGT to prevent both flame rushes and APC rushes. But, it can't sit on it's can the whole game defending either, and expect to win. One good strat that I've managed to pull off a few times (on the rare team oriented game), is to, again, make sure to have two med's or mammoths at your base entrance, ready to fend of flamers (and at least two techs ready to repair the AGT). The second step is to have a well loaded APC ready to bolt at the first sign of a flame rush. For this to work, you HAVE to wait for the flame rush. Once you see the tanks coming, take off toward the enemy base. If the flamers are coming in from the top, go the side route. The reason this often works is because Nod usually spends so much energy putting the flame rush together that their attention is usually on it, and not on their base, so it gives a good window of opportunity for rushing. Again, this tactic is rarely usefull, as it requires too much coordination, and, I'm sure if you could organize your team that well, there would be plenty of better strat's than this you could arrange.....