I think whether a given type of battery will work in the ML depends whether it will fit, has enough CCA's, has sufficient reserve capacity and whether the alternator in the ML charges within the requirements of the battery. You may want to consider an AGM battery. When my ML's battery died after 4.5 years I just got a very large Interstate Battery the MTP-93, a regular lead acid battery because it was relatively inexpensive and I'd had no problems with the previous OEM lead acid battery.

I've used GEL batteries in the past on boats and they are expensive, sensative to recharge voltage, and have low CCA's, I believe they are mostly for deep cycle applications.

Here's something that may interest you that I copied about AGM batteries of which the Optima is one brand.

AGM (Absorption Glass Mat) sealed battery technology was originally developed in 1985 for military aircraft where power, weight, safety, and reliability were paramount considerations.
AGM battery technology has continued to develop and offer improvements over other sealed battery technologies. AGM technology has become the next step in the evolution of both starting and deep cycle sealed batteries for marine, RV, and aviation applications. This "next generation" technology delivers increased safety, performance, and service life over all other existing sealed battery types, including gel technology.
In AGM sealed batteries, the acid is absorbed between the plates and immobilized by a very fine fiberglass mat. No silica gel is necessary. This glass mat absorbs and immobilizes the acid while still keeping the acid available to the plates. This allows a fast reaction between acid and plate material.
The AGM battery has an extremely low internal electrical resistance. This, combined with faster acid migration, allows the AGM batteries to deliver and absorb higher rates of amperage than other sealed batteries during discharging and charging. In addition, AGM technology batteries can be charged at normal lead-acid regulated charging voltages, therefore, it is not necessary to recalibrate charging systems or purchase special chargers.