Ground fault interrupt breaker
You must use the same neutral as the hot or every time you plug in the gfci will trip. GFCI'S work by detecting an imbalance from the hot and neutral. To find the neutral trace the hot wire back to where it enters the box if 1 cable entering the box black, white, and ground that white is the neutral you are looking for, remove that one from the buss and install it in the gfci breaker if only 1 white on the breaker then the white from the breaker to the neutral buss. if your gfci breaker has 2 white wires the white with a black stripe is usually the load neutral. tie white black stripe with the white you removed from the buss  the 2nd white from the breaker goes to the location on the  neutral buss you removed the wire from or one that is closer to the breaker but it needs to go to the buss now put the hot (black) on the hot side of the breaker , turn off the breaker plug it in then turn the breaker on. Breakers should always be in the off position when removing or installing for safety. 

Added from deleted answer info,  having multiple wires is a little tougher but not two hard, first turn OFF every breaker that wire comes in from that pipe or the main, this will keep you from being shocked when trying to find the correct one. With the power off on every hot wire that enters the pipe Re there equal numbers of hots to neutrals? If there are less neutrals jump to the bottom where it says MWBC)
go to the outlet that you are trying to protect with the GFCI breaker. Pull the outlet, remove the neutral and hot from the outlet and put a wire nut on the 2 wires. Now go back to the panel and use a continuity tester or ohm meter and remove the neutrals that come from that pipe and check for continuity from the hot for the gfci that is turned off and the neutrals. On of the neutrals should have a very low reading less than 10 ohms for most homes. Once you find the correct wire showing a very low value mark that neutral and tape them together hot and neutral. Now if only 1 neutral pull the wire nut at the outlet and separate the hot and neutral and re check the hot and neutral it should not show continuity or now a very high resistance value if so hook the neutral to the gfci breaker explained above install the outlet now time to turn on and it should work. 

MWBC if there are less neutrals than hots you have multi wire branch circuits where a neutral is shared by 2 hots on opposite hot legs (L1/ L2). You can use a double pole GFCI and hook the 2 hosts and neutral to the new double pole gfci or pull a new neutral pulling a new wire won't be that hard in pipe but dpGFCI's are expensive and you already have one. The hard part here is finding where these 2 hot wires and neutral go. It will usually be close to the outlet we started with if this was the first outlet on that circuit, once you find where the hot and neutral split at that location I would pull in 2 New neutrals back to the panel using the original neutral as a pull string (you could reuse the wire you pull out by attaching a string and 1 new neutral. But I usually just pull 2 for cases like this). Now you have a neutral on each hot so your gfci will work making sure to get the correct neutral with the hot to do this without thinking I will pull a gray and a white both are legal neutrals but in large facilities gray is usually for "other voltages than 120" like 277 or a 230 3 phase system,,,. In residential it helps to identify multiple neutrals in the pipe., hope this covers every thing, if not comment below this and every one will try to help clear things up for you.