Both to some degree, realistically. I used an old collander as a signal reflector for a wifi dongle on the end of a USB extension cable and was able to boost the signal up to about 4x, or maybe half the range of the purpose-built and highly directional Yagi antenna I eventually bought to replace that kludge.

I have tried that. I have a dish taken from a directional WiFi antenna. When placed behind the gateway, it sometimes increases speeds, sometimes hurts speeds, and sometimes does nothing. I found it a bit too inconsistent, and a bit too ugly, to be used permanently. If I had a proper mounting solution, I might have gotten it tuned just right, however at that point I would rather just buy and mount external antennas to hook into the gateway.

My exact deployment today actually doesn't even have anything behind the gateway. That is just because for my specific case, all the towers it can reach are within a roughly 90 degree field of view. To block the bad ones, I really only need to block off a few sections of the window it's sitting near.