These are a very simple and quick knit (hence the freeness). The pattern includes instructions for mitts with a fitted wrist (using decreases and cables) or a simple mitt with no shaping. Also included are instructions for an afterthought gusset thumb.

Notes on Thumb Gusset:
It has come to my attention that I have rather big thumbs (all the knitting maybe?) and the gussets as written are turning out a little baggy. Here is a mod for smaller thumb gussets:

For best fit, try on the mitt at frequent intervals during the gusset process.
When the gusset is deep enough to fit your thumb (this will happen somewhere in the repetition of rows 4 and 5) move right along to the “Next Row” section and repeat those two rows until you have used up all of the picked up stitches.
Note, if you repeat 4 and 5 fewer than five times, you will end up repeating the 2 rows in the “Next Row” section more than six times. For example, if you only repeat 4 and 5 three times, you will repeat the next section 8 times.

Many thanks to the knitters who have commented and included their fixes!