Otherwise you might hit sudden breakage if glib:3 enters the tree, although unlikely because of gtk+:2 dep, but let's not test it.
The proxy maintainer did not do this as he did not spot it, this will be done as part of the next version bump as requested.
I will fix this in two weeks.
Thinking about it again, there is indeed no need to wait for a revision bump. Slotted the rest as well... :) + 21 Apr 2013; Tom Wijsman <TomWij@gentoo.org> osm-gps-map-0.7.3.ebuild: + Slotted dependencies to avoid breakage, fixes bug #464240.