Is the ADM Mass for a boosted black hole $M$ or $\gamma M$?

What ADM defined was an energy-momentum vector, which can be interpreted as the energy-momentum vector of an isolated object in the frame of reference of a distant observer. (The notion of a distant observer only makes sense because we assume an asymptotically flat spqcetime.) This energy-momentum vector transforms as you would expect, so yes, the energy component becomes $\gamma M$ if you're not in the rest frame of the object. The ADM mass can be defined as the norm of this four-vector. For a more detailed discussion of this, see Wald, p. 293. The sentence that explicitly answers your question is the one beginning with "Furthermore, the Einstein evolution equations..."