On one inside face of the angle I create a sketch for one end of the gusset. Since I extruded the angle symmetrical about the centerline, it makes it easy to create this sketch symmetrical about the centerline too.
On the other inside face I create a matching sketch.
It's then just a simple loft between those sketches.
There are multiple ways to create this part. Maybe an even better way that what I have done. But see if you can recreate these steps on your own.