Okay, to save time, timeline issues:
We can use SG's call as one fix point: That is 5:14.
We have one call at 5:22, 8 minutes later and that is, as far as I understand what the Foreigner said, BB. GC's call was in between.
(I "structure" this a bit, so also Just K can read it)
So, we have about 8 minutes. In this eight minutes, GC met SG, he met MP, he called the police. So the chances, that BB walked her dog somewhere else than where she said, she saw SG without seeing MP would be very slim (I leave out UFO technology here)
The same problem would come up wither Gerontion's idea, GC told a lie to protect a certain elderly co-resident of OB (who is btw younger than GC). I can see, the crisis meeting of the OB residents, the discussion necessarily to convince everybody, the fights and struggles, all in about 6 minutes of time. Yes, I can see that (for those who didn't notice, I follow the open mindedness theory and word my doubts as absolute and political correct lie to the opposite)
Sooooo ... there was no time for BB to meet GC anywhere, not intentionally, not by accident before all three calls (SG, GC, BB) were made anyway.
There was no time for a certain elderly resident to make the call instead (which would be on his phone records and his phone records were checked by LE) and run over (one one leg) to GC and talk him to go to the gate, and open it for the police.
MP was also pretty usre, her met GC and GC was pretty sure, he met a guy with a black SUV and somewhat Asian appearance. So if G. claims, GC told a white lie (painting him a liar once more), he also says, all those other things (GC saw GS, he saw MP) were lies.