Currently, items in the Glossary pane are sorted by the length of source terms, and there's no way to change that. In Auto-Completion settings for the glossary hits, though, there's an option to sort items alphabetically.
In many cases it would make much more sense if the Glossary pane items were sorted in alphabetical order, or in the order in which they appear in the current segment.
Edit:
Duplicated with https://sourceforge.net/p/omegat/feature-requests/1241/
Change is proposed
https://github.com/omegat-org/omegat/pull/285
A ticket is duplicated with #1241 alphabetize terms in glossary pane
Previous discussion here https://sourceforge.net/p/omegat/mailman/omegat-users/thread/001401d2c3f0%246cbc1ac0%2446345040%24%40.../#msg36851792
Diff:
The fix was merged.