GPS time is NOT kept in phase with UTC, so when a second is added to UTC, it is NOT added to GPS time. So there is no glitch at all in GPS positions when a leap second is added to UTC.
GPS time was zero at 00 00.0000000 on 6th January 1980 and is now 18 seconds ahead of UTC.
So as I'm typing this:
UTC time was: 2017-01-01 09:33:23
GPS time was: 2017-01-01 09:33:41
People who use GPS at a professional level, which would include masters of ships (other than perhaps recreational sailors) as well as surveyors, geodesists and many engineers, are well aware of this.
Interestingly (or not?), when I re-triangulated part of northern Brazil in 1985 using Block I GPS, I found a 55m bust in their primary triangulation network around Macapa....which IGBE (Brazilian equivalent of OS in UK) took some convincing was real.