Basically, the BIG question is: where does 120 lbs Bruce Banner get the mass to instantly turn into 1,000 lbs of Hulk?
First, let's look at the gamma effect. It was called a Gamma Bomb because it generated a great deal of Gamma Radiation when the bomb detonated, BUT the real effect occured on a quantum level. If you look into quantum physics, you'll see that empty space is constantly boiling over with quantum particles that appear collide and disappear every nannosecond. The gamma effect was designed to create interatmospheric instability causing these particles to appear much more energetically so that all organic matter within the "blast range" would be immediately destroyed by the heavy energetic interactions. However, unlike the atomic or neutron bombs, once the effect ceased, there would be no leftover radiation or fallout, and the cities, though scoured lifeless, would be immediately inhabitable. Banner understood that he was creating a devastating weapon, but he hoped that it would ensure that future nuclear war did not leave the planet toxic with lethal radioactivity.
Now, when Bruce Banner saved Rick Jones' life, he was certain that once the effect hit him, he would be immediately reduced to nothingness on a quantum level. What he hadn't considered was that his years of research had brought him into contact with the gamma effect so often that his blood had become, in essence, a gamma effect generator. His body's atomic structure was becoming highly unstable and this could be seen in his neurological system as well. Ironically, if Banner had not been caught in the blast, he would have most certainly died from radiation sickness.
When his biological gamma effect was struck by the massively more powerful Gamma Bomb Blast, the two fields interacted and Bruce's body generated additional quantum particles modeled after the unstable cells in his body. The result was that he obtained a supercharged and much more massive physique than thought humanly possible. This effect was also hardwired to the rage portion of Banner's brain so that deep anger, repressed since childhood, triggered the quantum level changes. Also, as his body grows, it delivers a feedback loop to the brain upping the rage levels. Only deep wells of sympathy and compassion in Banner's natural personality act as subconscious brakes and prevent the Hulk from becoming an outright killer. Once the effect runs its course, the positive feedback eventually reaches a chaos point where the mind can no longer keep focus and the nervous system reverses the transformation allowing quantum particles to collide and cancel each other out until the banner form returns.
It is interesting to note that, on a cellular level, Banner and the Hulk are completely "new" men after every tranformation since all the old cells are eliminated by the gamma effect.