Use a wallpaper-perforating tool (Image 1) such as the "Paper Tiger," which is a roller with small spikes that will perforate the paper (Image 2). This will allow the solvent to get behind the paper and dissolve the adhesive more readily. Be careful not to press too hard so you don't perforate the wall surface. If it happens accidentally, you'll need to fill the nicks with spacking compound.
Apply the gel or other commercial solvent with a paintbrush, and give it some time to soak in and loosen the wallpaper (the label should specify how long). A less expensive solvent option is warm water mixed with a few tablespoons of vinegar in a spray bottle.
After the solvent has a chance to soak in, start working at removal at a seam or edge of a perforation, scraping with a wallpaper scraping tool, plastic putty knife or drywall knife. Keep applying the solvent and continue scraping until the wallpaper is removed.