I've been looking at delta flights coming into the airport from Atlanta. They all mention either a gate B12 or in this case, a gate B13. One quick look at the airport terminal map though (and even the search feature the map has) results in nothing found for gate B13 or B12. Whats going on here?

I believe that they move B13 back and forth with A13

From Google Maps satellite view, it appears that A13/B13 share the same stand. B13 is perhaps the international arrival gate, while A13 is a gate that people can walk to (departures), so that's the gate Pearson depicts on their maps. B13 is painted on the tarmac and is visible on Google Maps.

A13/B13 are the same gate. This arm of T3 is used for both US and domestic departures. When used for US departures, the gate designation starts with A, but for arrivals and domestic departures, it is designated B. I don't believe gate 13 is ever used for domestic departures, but B17 through B19 can swing between US and domestic departures, and would be indicated as A17..A19 in that case.

That�s how it works in T1 as well. At T1, when the proceeding letter is �D� it�s for domestic, when it�s F it�s for transborder and E is for international. There are some gates that can be used for either domestic or US departures, so those gates (mostly or maybe all in the low-mid 50s, I think?) start with D or E depending on the flight. There are also some E gates that can be used for transborder departures and so are E/F gates - they are in the 70s - though I�m not still if they use them for both anymore - they were typically used for US departures in the peak morning bank, and than international all other times. However, not sure if they ever use them for transborder since they opened the other side of �the barn� - those gates in the 90s that United typically uses, AC too.

and if Canadian airport history ever becomes a Jeopardy category, might be good to know some history - when the �new� T1 opened, the gates were all 3 digit numbers and didn�t use the letter designations ahead of the two-digit numbers until later (T3 might have been the same way, I�m not sure). These numbers indeed are still referenced - for example, if you look at flight status on United�s website for any inbound flight to YYZ, they show those old 3 digit numbers as their arrival gate - the departures show the �official� �F� numbers. For example, today, UA 5450 from ORD shows the arrival gate as �162� - that aircraft turned as UA 5578 back to ORD, leaving from gate �F62�.

YVR has a similar setup with at a good number of gates that switch between D & E depending on whether they'll be int'l or preclearance trans-border. There are also some (3?) gates that could switch between domestic and int'l.

I remember that! They switched while I was away for a few days. My outbound connection left from a 3-digit gate, and later that week for my inbound connection there were lettered gates everywhere. I thought I had switched to a parallel universe. Maybe I did.

From a pilots perspective, 3 digit gate numbers are still used.

48, 49 can be used for international arrivals followed by domestic departures.
50-52 can be used for either int'l or domestic.
There are 2 or 3 B gates that can be used for international arrivals followed by domestic departures.