The "ghosting" you see is because the lettering has been protecting the gelcoat for as long as it's been there while the gelcoat not protected by the lettering has been weathering in the sun and atmosphere. In otherwords, it's "like new" while the rest of the gelcoat is possibly several years old.
You can't really make the lettering look like the rest of the boat, you can only try to make the rest of the boat look "new" again like what was underneath the lettering.
How you do this depends on the condition of the rest of the boat's gelcoat. Start with a gelcoat restoring product. If that doesn't work, try rubbing compound. If that doesn't do it, wet sanding, followed by rubbing compound and wax is what you'll have to do.
Before you get started, read up on compounding and/or wet sanding to make sure you know the proper techniques.
You don't have to do the entire boat, you can feather the repair or you can find a natural border such as a rub rail.