Last I checked it was not possible to use the rotate or scale functions to change the positions of vertices in Mesh Edit mode. There's currently no way to access the rotate or scale tools when in Edit Mesh mode, unfortunately. You can only adjust the position of vertices using xy translation with the mouse/arrow keys.
I had pretty much exactly same problem a while ago. Nate added an entry to the issue tracker, but it hasn't been addressed yet.
Rotate and scale vertices in edit mode · Issue #60 · EsotericSoftware/spine-editor · GitHub
The only thing I could suggest would be to change the rotation of the glove in the underlying image. Open the image in photoshop, rotate the glove the required amount, but keep the image's width/height dimensions the same (very important). Save the new rotated image in your project/images folder. Then within Spine, you can change the image file that the mesh uses.