I've removed a lot of engraving, but mostly on brass, bronze or silver. The same principles would more or less apply to steel.
The engraving is most likely diamond drag done on a pantograph. Diamond engraving doesn't remove any metal, it just displaces it. Picture a V-shaped ditch, with the displaced metal as the raised "ditchbanks". The engraving will be about .003" deep.
First step is to burnish the surface to try and work some of the metal back into the cut. Xacto makes little burnishers that fit their knife handles. This works well on soft metals, but not so well on stee, but it's worth a try.
Next step is to remove the engraving. I would start with about 110 grit wet-or-dry and work up to maybe 600 grit. A block of styrofoam makes a dandy sanding block for irregular surfaces. Make sure all traces of the engraving are gone before polishing or final sanding. A buffing wheel will NOT remove engraving.
If the gun is stainless, just experiment with different grades of sandpaper until you match the finish. If the gun is blued, it will have to be repolished. You can either leave that to a gunsmith who does hot bluing, or you can do it yourself.
Use an unstitched buffing wheel, and probably white rouge (ask your jewelry supply, gunsmith or knifemaker). I use red rouge to start, and then white to finish, but I work with brass, and that won't work on steel.
When you get the frame highly polished, clean it thoroughly with mineral spirits or acetone, and then cold blue it. Hot bluing is much preferred, but cold blue will get the job done if your polishing job is good.
Moving this to Gunsmithing and Repairs.