Gel is a broad label, often used on foam toppers. Phase-change material, or PCM, stores and releases heat during a phase transition. Those descriptions do not establish the performance of every finished mattress product.
A mechanism is not an overnight result
Outlast explains its phase-change mechanism : its material absorbs heat as its state changes and releases heat as conditions cool. That is a finite thermal-buffering process, not refrigeration or a promise that every PCM pad stays cool all night.
For a fair comparison, look for evidence about the exact finished product, the amount and placement of the material, and the conditions of any test. Do not transfer a manufacturer’s result for a different fabric or duvet to the topper in your basket.
Construction still controls the buying decision
A gel-foam topper changes cushioning because it adds foam. PCM can be used in different constructions, including covers, so it does not automatically mean a thin product that preserves firmness. Check the actual layer thickness, cover and care instructions.
If you want adjustable temperature, compare an active system instead. If you want different cushioning, evaluate that as its own feature. Neither gel nor PCM is declared the universal winner for night sweats or waking hot. See the cooling bedding guide for the category distinctions.
See Also
If you are still weighing both sides of this matchup, keep going with Thin vs Thick Cooling Pads: Decide How Much Feel to Change, Washable Cooling Mattress Pads: What to Look For, and Mini Sound Generator vs. Bulky Sleep Sound System.
To widen the decision beyond this head-to-head, What to Look for in a Bedroom Humidifier for Large Rooms and Best Mattresses for Every Sleep Style provide the broader context.