I found a neat fingerless mitten pattern on Ravelry, CSM Fingerless Glove by Melody Cowan. It has a real gusset and gradually increases to fit the thumb instead of the rectangular increase with an abrupt joint for the hand. But this method requires lifting extra needles to clear the cam when you knit back and forth. Basically, when you knit back and forth on a circular sock machine (when doing heel and toes), you need to raise the needles (at least 1/3) out of work to clear the cam to go the other way. But for a real gusset, as you put more and more needles back in the work, you will be running out of room to clear the cam. The trick is to alternately raise needles (out of work) on one side and lower the needles (in of work) on the other as you knit back and forth.
Here are some pictures of the mitten.