How do I get rid of hole in Thumb Gusset?

I am making my first pair of mittens. I am using the 2 needle method. When I got to the part were you joing the front and back it left a hole in the thumb gusset. How do I fix it so the holes is no longer there?

You can pick up an extra st there when you knit the thumb sts, or use the tail from the thumb yarn to close it up.

Thanks for your help.