Ah right. The drum sequencer doesn't output velocity CV, only gates for pads 1-8 (the bottom row from left to right respectively). The gates are high for the time the drum gate is open for and the high voltage is 10v. Can't see where you would plug gate and velocity CV's into the Mutant Brain though, the only jack points on it are all outputs.
The MIDI velocity is the standard MIDI velocity message that accompanies the MIDI note-on message on the channel you set the drum sequencer to and has no CC number assigned to it the same as a regular drum machine sequencer MIDI output would work. Without looking at the Surgery app I'd assume that you'd use the it to assign various MIDI note velocities and gate times to the many outputs on the modules. If that is the case, connect your BSP to Arturia's MIDI Control Centre software and take note of what MIDI notes you have assigned the BSP pads to send, then go into the Surgery app and assign them to a velocity and gate output.