There has been situations where users are missing from Global Address List.
I myself have faced this a few times now. In our environment it only happened when user was rehired.(re-enabled) -->(this happened rarely but when we researched a bit ,finding was --> user was re-enabled)
So thought of sharing the solution which is very simple but is not known to many.
just run the following command
Update-GlobalAddressList -identity "Default Global Address List"
Better to run this command during off-peak hours as for big enterprises it can take long time to finish.
There is one other situation Where user has different email address displayed in email tab(exchange console) & different in email tab of AD console.
We first resolved that issue & than ran the above command to update the user in GAL.