Gas fireplace insert won't stick to marble fireplace
First and foremost, I would attempt to locate the instruction manual for your unit. You may still have it somewhere in a junk drawer or similar, but almost just as easy, you should be able to look it up online or by contacting the manufacturer.

Many of these inserts are mounted by a series of fasteners from the structural interior of the where the unit is to be mounted, either directly to the gas insert, or to a bracket that will support, attach and secure the insert mechanical. To date, in the US, I cannot imagine any that are specified to be installed with any kind of adhesive. This just wouldn't make sense for servicing.

IF you unit is literally falling out of the wall, there is a very good chance for damage to the gas supply line and/or electrical ignitor and thermostat system. I would personally not feel comfortable with anything short of a licensed and certified tech to come and repair/install my unit.

If gas supply, electrical and HVAC are out of your area of expertise, please see a professional.