A lot of conflicting information has been published regarding the incubation of Gila eggs and many breeders have had difficulties bringing eggs to full term. It is important to consider three factors that affect the environment of the developing Gilas—temperature, moisture and oxygen tension.
Suggested temperatures for incubating Gila eggs range from 26 - 29 C (79-85 F). It has been suggested that incubating them over 29 C (85 F) can result in failures, however, I have incubated a clutch up to 31 C (88 F) with a 100 % hatch rate. I have also incubated them as low as 25 C (77 F) with complete success. I currently incubate at 28.9 C (84 F).
Most breeders use vermiculite as an incubating medium, although other substrates have been used. Suggested water-vermiculite ratios range from 1:1 to 1:4 (by weight). I am of the opinion that excessively wet incubation medium is harmful to Gila eggs and is responsible for many failures.
Although Gila eggs should not be exposed to excessive contact moisture, they do need to be exposed to adequate humidity.
I currently recommend incubating in a 1:2 ratio (by weight) of water to vermiculite. In order to provide a stable environment for the developing eggs, it is helpful to have a large mass of incubating medium. I use a 4 liter plastic container with 300 gm of dry vermiculate and 150 gm of water. I have very limited ventilation in the container which I increase slightly for the last month of incubation.
In my experience, and contrary to popular opinion, successful incubation of Gila monster eggs is not difficult. If the eggs are healthy going in the box and some very basic environmental requirements are met, healthy monsters will hatch.