You could paint the plate trying not to get too much buildup on the letters, then sand in one direction with a fairly fine grit paper on a hard block. The idea is not to sand the deeper parts of the headbadge.
I've heard you can use vaseline to coat just the lettering and paint, then wipe off the greased up areas...but how well that will work on the fine edges might be an issue.
Or, use a fine brush to paint only where you want color.
Or..if you really are good with an exacto knife and masking tape, mask it off then remove the masking right after spraying the paint..(Sometimes it works, sometimes you pull off chunks of the paint.)
If the GT is just printed on the aluminum and not raised at all.....you might do better to have someone make you a new decal.
Some combination of the above should work, assuming the letter surface is well defined and fairly flat other than the built in curve of the headbadge so it fits the headtube.
Try it out on a junk headbadge or something similar first to get your technique down.