High-speed signal across plane gap
That will provide a signal return path, but will also AC connect your two grounds.
Usually when I see split grounds, it's to prevent noise from one ground from affecting the other one.  You should take a look at what's near to the connection point and avoid having any noisy or sensitive circuits in the area.
Personally, I like to keep grounds together and try to route signals to localize noise--which also helps EMC--but every design is different, so you have to make your own call.