The only additions to Lawrence's response is to be sure you are in the "spray transfer" mode. You might even conside using the same gas mix as is being proposed for GTAW. The addition of helium will increase the heat input and aid in obtaining good fusion in the root of fillet welds and grooves on the thicker sections.

Use the largest diameter electrode your equipment can handle. Small diameter electrodes tend to "bird cage" easily and more passes are required to fill the groove or to make a large fillet welds. More passes, more thermal history, more chance of overaging, and diminished mechanical properties. Clearly, the thickness of the members being welded play an important part in selecting the proper electrode diameter.

Minimize interpass temperature and the time at temperature by using fans to blow cooling air on the weldment between weld passes. Don't use a jet of compressed air to cool the parts!

My experience is that those welds made with GMAW will have better mechanical properties than will those made with GTAW provided the same filler metal alloy is used for both processes.

Best regards - Al

Before switching to MIG I would look at the inspection requirements of the weld and the overall joint. Mig ins't so great for a weld that requires many start and stops that need to be leak proof. Other wise I would suggest using zirconium rather then pure tungsten and a preheat of the parts if at all possible.
We used to sandwhich a 1/8" aluminum plate between two 5/8" aluminum plates. There was a large V prep all around and the welds were machined flush after welding. All plates were roughly 4" x 6" in size and it had to be leak tested after welding. I wanted to switch to mig, but it wasn't feasable with this set up. I think it would be difficult to tie in all the corners withou any leaks. We tacked all the parts and left them on a hot plate, then they were hot for welding and the job went smoothly with a very low reject rate using tig.