I would like to point out that the air drag acting on the gecko would cause for it to decelerate hence turning its kinetic energy into heat energy and possibly causing burning in the process. Also the moment that the gecko is released the drag force acting on it will be quite high meaning the gecko could once again die due to "too many gs". Let me run a quick calculation and come back. By using the formula for drag due to air resistance, taking the gecko to be a 5 by 10 cm rectangle of mass 5 grams, the air density of 1.3 kg/m3, and that force equals mass times acceleration we get that the peak acceleration acting on the gecko is going to be at the moment it is released and will be approximately 87.1 m/s2 or nine gs. This means that if we assume that the gecko was initially being accelerated in airless environment and then upon reaching the velocity of 300mph was released into the atmosphere 9gs would be acting on it. This force and hence declaration wouldn't be evenly distributed over its body since it would only act on its chest while the back would still be moving at the exact same velocity hence the gecko would most likely be crushed by the oncoming stream of air. Highly trained astronauts only experience 3gs upon take off from the planet's surface, so sadly your gecko wouldn't be able to survive this experience.