The purpose of the gusset is to make room for your thumb because your hand isn't perfectly square, the thumb sticks out.
(please excuse my abominable paint skills)
When you're knitting the gusset, you'll mark the spot where the thumb will be with stitch markers. Outside of those stitch markers, everything is knitted normally. INSIDE the stitch markers, you're going to do increases along the edges.
This is how I like to do the gusset:
R1: K2, place marker, K2, place marker, K rest of the row
R2: K2, slip marker, inc 1, K2, inc 1, slip marker, K rest of row
R3: K all, slipping markers as necessary
R4: K2, slip marker, inc 1, K4, inc 1, slip marker, K rest of row
R5: K all
Continue this pattern until you have 14 stitches in between your markers. Have scrap yarn ready.
Next R: K2, remove marker, K1, slip 12 stitches onto the scrap yarn, K1, remove the other marker, continue knitting.
Finish the glove, slip the 12 thumb stitches from the scrap yarn onto your needles, pull up 2 more stitches along the top of the hole, and knit in the round until the thumb is as long as you want.
If you'd like more help, send me a message!