The gain these two preamps provide is very different. Not sure but I think the dartzeel is +6db and the gat is around +16 dB. This is usually as they are designed to be paired with their amp from the same brand. My Nagra PL-L has a lot of gain and I reduce that quite a bit pairing it with a SS amp for my 2 channel HT system.
darTZeel preamp provides max +11 dB gain.