who knows what tag this is?

it is about 1.4 inches in diameter

enlarged and enhanced

It is a Canadian Military Tag, GNR which is the acronym for his rank which was Gunner, last name Prior o and possible forename initials of W. B.

K25138 was the serial number assigned to the enlisted consisting of one letter and 5 numbers and probably was issued in the second world war. The K or letter denotes the area where the person enlisted.

  I say that as my fathers serial no. started with K as well and was followed by 5 numbers. I referenced the serial numbers issued in WW1 by looking my grandfathers serial number which consisted of 6 numbers only suggesting this serial  number may have been issued in WW2.

CDN at the bottom of the tag probably references him as a Canadian and the other two letters I cannot make out.  

Using the serial number you may be able to have the military or canadian archives provide to whom this tag belongs or find someone who can.

Diana Farrell