My spouse uses a Mirrycle mirror with GP1 grips (where I choose to use bar-end grips with my GP1 grips).
I actually adapted a dropped-bar brake-lever version of this mirror to fit *some* of the mirror assembly to another manufacturer's bar-end plug.
Note that Mirrycle mirrors are pretty robust, they are not indestructible and so it helps if they are able to rotate / pivot a bit when knocked / bike is dropped so don't do them up really tight so that they are completely rigidly mounted.
My adapted version lost one degree of freedom and so the specific flat-bar version may be more suited
although this means that it sticks out further
https://www.mirrycle.com/mountain_mirrycle.phpShe also tried the Cateye bar-end mirror, but it is too feebly clamped and vibrates too much.
You might alternatively consider one of the B&M mirror variations
http://www.bumm.de/produkte/mehr/fahrrad-rueckspiegel/cycle-star.htmlI have no direct experience of using these, but some of my Dutch relatives do use them (but not with GP1 grips) and like them.