Sooo...

I have recruited all possible followers, they were in the presidential suite in the lucky 38 ( I didn't get the achievement but that's another story) and here I am with cass and rex. So I dismiss cass inside the suite and go to Gannon to tell him to follow me so I can get the quest "for old Lang syne"...
And as soon as I fast travelled, Gannon was gone. Just gone, can't find him any more, can't recruit a different companion, nothing.
Is there a way to solve this other than loading a previous save? Cause my last save with cass is about one and a hake hour realm time before Gannon was gone.... So I would have to redo a lot of things and I would like to evade that...

try waiting or slepping for more then 5 hours worked for me, he just pops up out of thin air, I went into hoover damn fast travel to the strip and he was gone (still has the perk for better stimpack though)
so I tried re-loading my last saved game, nothing still gone then I just waited to see what would happen... and like magic he just stood there!

Ok... Thanks, it miraculously worked. Waited for 24 hours and he was back! Oh and...

I fast travelled to the crashed vertibird... And back to free side which is where I waited.... Upon returning, Gannon started talking to me about the vertibird...
Srsly, bethtesda.... How did you manage to make that happen?!?! WTF?!?

similar problem and now cannot get for auld lang syne to start.... any suggestions?