I have used both but never compared them side to side. Needless to say they're both good. The Elmarit v4 is a near-symmetrical, retrofocal design, which makes it very flat and excellent on digital (it produces little artifacts). The GR lens is non-symmetrical (I'd say it's closer to the pre-war Biogon) with exotic elements (two aspherical if memory serves). Not as flat, vignettes more, but offers a bit better resolution at larger apertures. It's very modern, comparable to the ASPH Elmarit, but not as clinical.
This Japanese website, complied over a periold of two decades, offers a comprehensive overview of almost all native wide angle lenses you can use on a M: http://geofoto.web.fc2.com/leica-lm1.html
, including all versions of the Elmarits and of course both the 21 and 28 GR lenses in LTM. Try Google translate it. Low resolution example shots taken from the host's balcony are offered...at least they do help in grabbing a rough idea on how the lenses compare.