receiving/retransmitting
If you use a directional (uni) then you may not pick up your AP signal (of course depending on which direction your aiming it). If you put to high of a gain antenna it tends to flatten the signal so it may not reach as far down into the second floor. Think of a donut after steppin on it. If the 6 dbi covers the second and first floor good then I see no reason why not putting one on the repeater. It would probably allow you to move it a bit further down the hall. And yes, the gain is for both transmit and recieve.