Why do you neeed to remove the glue ? in most cases you can cover with a new substright
The best way is to use a product like Bean-e-doo mastic remover. It is a liquid that you pour over the around and let it sit for around 12 hours.
Heat gun should work perfect for that and scraper.
Before you do anything to it, like scraping, chipping, heating, or sanding, have the mastic tested by a lab for asbestos. Many of those old “glues” had significant quantities of asbestos fibers added and they will become airborne and create a significant health hazard if improperly removed.----Then use a very low grit sandpaper and sand the heck outta it…worked for me—