Which Category, ATM or NTM, Is Likely to Have a Higher Gamma Value?

The At-the-Money (ATM) option is typically the point of maximum Gamma. Gamma is the rate of change of Delta, and this change is most rapid when the option is transitioning from OTM to ITM or vice-versa, which happens exactly at the ATM point.

Near-the-Money (NTM) options will have very high Gamma, but the absolute peak is generally considered to be at ATM.