There are a couple ways I've increased stitches for thumb gussets in mittens or gloves. The most unusual method I ever used it was for Nancy Bush's Vanalinn gloves in A Gathering of Lace. The pattern stated to do a YO just on the palm side of the gusset. To me it really looked like holes in my knitting.

For my most recent Selbu gloves I strayed from my usual method and used a M1 left leaning increase on the right side of the gusset and a M1 right leaning increase on the left side of the gusset. While it is more noticeable, I'm not sure that is an asset in a thumb gusset. M1 increases look best if done every three rows or more.



I'm back to my usual method which is to do a M1 that slants to the right on the right hand side of the gusset and a M1 that slants to the left on the left side of the gusset.



This is the Herrevott No. 4 mitten from the Norwegian Selbustrikk book done in Nature Spun sport weight and size 0 (US) dpns. This results in a snug knitted fabric which I prefer for mittens.