Since GSX Level 2 is FREE to use at FSDT airports, the only possible reason you saw that message, is that you have an AFCAD conflict caused by another scenery, so GSX doesn't know you are on an FSDT airport, because it hasn't loaded the FSDT AFCAD.
So, check if the AFCAD in use by GSX (from the scenery customization page), is the one that comes with KIAH. If it's not, the name of the AFCAD listed there should give you an idea how which scenery is conflicting with it.