Try using compressed air, either by air compressor or canned duster.

Put the nozzle between the handlebar and the grip with the grip partly on the bar and then blow. The grip will kind of inflate/ widen, while doing that pull/ twist the grip down the bar.

Be sure to cover the holes at the ends of both before blowing otherwise the air will just find its way out.

If you go the duster route, be sure to not turn the can upside down or sideways in the process.