Engineering the Dirac gap in magnetic topological insulators (TIs) requires understanding how magnetic atoms interact with topological surface states (TSS). Replacing Mn atoms in intrinsic antiferromagnetic TIs of the (Formula presented) family with other (Formula presented) transition metals offers a systematic route to study this interaction. Using first-principles calculations in AFM-z configuration, we show that full substitution of Mn with Ti, V, Fe, Co does not significantly alter the structure of TSS. The Dirac gap of Mn-, Fe- and Co-based compounds is in the 30–50 meV range whereas Ti- and V-based systems exhibit smaller Dirac gaps of approximately 10–15 meV. Hypothetical nonmagnetic topological insulators (Formula presented) and (Formula presented) with gapless TSS are also suggested. Variations of the Dirac gap size are found to correlate well with magnetic moments of atoms weighted by their contribution into the TSS.