Honestly, no urban outdoor furniture is completely immune to chewing gum, but the closest you will get is smooth, non-porous, sealed materials with as few seams and crevices as possible. If I were specifying furniture for a busy plaza, transit stop, campus, or parklet, I would start with powder-coated aluminum frames paired with HDPE slats or compact laminate tops. These surfaces do not absorb sticky liquids, and cured gum usually releases with a little ice and a plastic scraper instead of soaking into grain. Choose a smooth finish rather than a heavy texture, because textured coatings give gum something to grip.
Solid HDPE benches and chairs are another strong choice. They are non-absorbent, UV-stable, and easy to hose down. Stainless steel, especially 316-grade, works well in heavy-traffic or coastal areas because it is non-porous and can handle aggressive cleaning, though it shows fingerprints and can get hot in direct sun. Sealed or polished concrete is practical for planters, benches, and tables if the surface is properly sealed; raw, porous concrete is a magnet for stains and gum. Phenolic resin or compact laminate is excellent for tabletops because it is dense, smooth, and easy to wipe.
What I would avoid includes unfinished wood, porous stone, woven wicker, deep wood-grain plastic, perforated metal with tiny holes, and slatted designs with narrow gaps where gum and spills collect. Cushions are usually a bad idea unless they are removable, vinyl-covered, and specified for commercial cleaning.
Design matters as much as material. Rounded edges, sloped or crowned seats, sealed seams, and removable tops make cleanup faster. For gum, freeze it with ice or a gum-removal aerosol, then scrape with a plastic tool; avoid wire brushes that scratch coatings. For sticky spills, use warm soapy water or a commercial degreaser, rinse, and dry. In short, powder-coated aluminum, smooth HDPE, stainless steel, compact laminate, and sealed concrete are the practical urban choices, especially when the detailing leaves gum nowhere to hide.