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An increase in altitude by 100 feet results in a change in gamma radiation due to the effects of atmospheric thickness and density. As altitude increases, the amount of air (and thus matter) above the observer decreases, leading to less scattering and attenuation of gamma rays.

Typically, for every increase of approximately 100 feet in altitude, we can expect to see an increase in gamma counts, which is quantified in the context of the question as "GAMMAS." The standard value often used in these scenarios results in an increase of about 1 GAMMA for each 100-foot increase in altitude. This reflects the relationship between gamma exposure and altitude, where the decrease in atmospheric pressure and density allows for more gamma rays to be detected at higher elevations.

Therefore, the increase of 100 feet in altitude correlates directly with the expected increase of 1 GAMMA, making it the correct response to the question.