The GTI no longer exists. Aside from a good chunk of it being excised after the Hades Insident, the remaining parts were merged with Vasudan Intellegence and became the GTVI. Special Operations Command (SOC) is simply the "go between" agency between the GTVA miltary proper and the GTVI. Thus the reason why ace pilots (Alpha One) are brought in on such missions, while normal intellegence operations have their own pilots (Snipes).
Well, that's how I interperated it. In truth, there is no known connection between the GTI and SOC, and the GTVI may not even exist!