Gateway is switching between mine and someone elses

Xfinity MoCA is a security problem they won't address.  They use MoCA version 2.0 and it has no security.  It's Ethernet over coax.  It's for their TV coax connected cable boxes. 

For the short term, you can disable MoCA.  You may end up having to disable your neighbors first, or MoCA on several neig hbors gateways before you are actually logged into your own network where you can disable it.  Drop your wifi or Ethernet connection and reconnect between attempts to log into your gateway, or the next one on the coax to verify you actually shut it off on your gateway.  People do report it turning back on by itself sometimes. 

For an actual solution, you need an MoCA point of entry filter.  It blocks the higher frequency Xfinity MoCA uses from either leaking out, or your neighbor's devices from getting in either intentionally or unintentionally.  They are around $10 on Amazon, or you can order one from Walmart.  You can also get one at an Xfinity store if there is one by you, and people say they are free there.  Search for "MoCA PoE" filter to find one.

If you have no coax cable connected boxes yourself, you can screw the filter directly to the back of your gateway inline.  If you do have coax connected cable boxes, then the filter would have to go on the coax splitter that feeds all the jacks in your residence.  It can be a rats nest of cable in apartment buildings and multiple family dwellings sometimes, but the goal is to try to isolate your coax from your neighbors.  

Or just not use Xfinity equipment with security problems they won't address, and use 3rd party equipment without their problems. https://www.xfinity.com/support/articles/list-of-approved-cable-modems