The original gusset pattern.
First I cut off the seam allowances. I was taught that when drafting, or adjusting a pattern, to remove the seam allowances, to keep your seamlines true.
Next, draw two straight lines, point to point, crossing in the middle.
Slash through the lines, then overlap your pieces, however much you like. I took my gusset in 2 inches. As you can see, it makes the gusset about 3/4" longer, but I found that didn't really matter.
Trace over the adjusted gusset, then add your seam allowances back on. This is the finished adjusted gusset pattern.