I am particularly interested in measuring brain activity in the gamma frequency range – won’t the D400 distort my data?

No, absolutely not.  This is one of the key features of active noise cancellation devices, in that they syncronise with the local mains frequency whether it is 50Hz or 60Hz and will ignore components of the input waveform even if they overlap with these mains frequencies.  As a result, many researchers interested in signals within the gamma band will opt for devices like the D400.