On the USRP E312, how can I find the connected gps antenna? Will it pop up in dmesg?
You don't "find" it. It's not a device in the linux kernel sense.
It's simply the antenna used by the built-in GPS module to receive GPS; you don't "see" it under linux, just like you don't "see" the Antenna that you attach to your e.g. Wifi card (you see that card, though).
This question reeks very very much of the XY Problem; please feel very invited to ask a new question (please don't do that in the comments!) that describes the actual problem you're trying to solve.