Hi, I'm a new member who just discovered this old post when I came up against the same problem. You probably resolved it ages ago, but I thought it was worthwhile giving a slightly more detailed reply for others who come up against this.
I believe you were following the the pattern for http://www.straw.com/cpy/patterns/mikado-gloves.html
As a relative beginner to knitting, I found the instructions for the thumb vague to say the least, although the design is a lovely one and it's worth perservering with.
Redwitch's reply gave the clue I needed to work it out, although it took a lot of frustrated head-scratching and ripping apart and re-knitting before I finally grasped exactly what she was saying a couple of hours later.
I found the thumb gusset to take 16 rounds and I've written down the first few rounds so you get the idea. I've written in bold the extra knit stitches so it's obvious how the triangle increases:
Follow instructions for first 3 rounds, then:
Rnd 4: K1, P1, K4, P1, work to end of round in pattern
Rnd 5: (odd number rounds are increase rounds) K1, make one inc by P1 K1 in nxt st, K4, make one inc by K1 P1 in next st, work to end of rnd
Rnd 6: K1, P1, K6, P1, work to end of rnd
Rnd 7: K1, make one inc by P1 K1 in nxt st, K6, make one inc by K1 P1 in next st, work to end of rnd
Rnd 8: K1, P1, K8, P1, work to end of rnd
Rnd 9: K1, make one inc by P1 K1 in nxt st, K8, make one inc by K1 P1 in next st, work to end of rnd
Rnd 10: K1, P1, K10, P1, work to end of rnd (for me this was also the cable round, so make sure you're ready for that!! I'm not even sure if I understood how many rounds the pattern repeat took so watch out for this on your own work)
Continue in this way until you get to round 16. The knit stitches on the even rounds go up by 2 each time. Hope this all makes sense and saves other novices time. I've just finished the left glove, now got to reverse the above for the right!!!
Nice to meet you all, I'll be back soon!