Gel Mattress overall durability will depend on durability of its components. With untested product like this you will have greater chance to rely on provided warranty. Warranties are design to protect consumer against manufacturer’s defects for certain period of time, and majority of innerspring mattress warranties protect customers against excessive sagging, which is typically referred to as an indentation/imprint/depression greater than 1.5”.
Most warranties start with a full mattress replacement for an initial period of time, and change to a limited (pro-rated) warranty as years go by. While comparing innerspring mattress warranties, you can find full replacement warranties for as little as 5 years and as high as 15 years, with overall warranty duration up to 30 years.