What you were expecting to happen, and what actually happened:
I interpret the additional words ‘under or’ to mean any gem below where you are placing the gargoyle gem (otherwise these additional words make no sense….just delete them). However, if there is no green gem around the one chosen but there are one or more directly below (ie 2, 3 or 4 rows below etc) it does not poison any opponents.
This implies that the spell is not working correctly, or the text is incorrect and ‘under or’ should be deleted.
How often does this happen? When did it begin happening?
Every time
Steps to make it happen again
Cast spell, select a gem with no green gems around it, but with green gems 2 or more rows directly below it and you will see that none of the enemy get poisoned.
“Under” might mean “the gem that was in the place the gargoyle gem was created”. So basically:
1.) Select a gem.
2.) Poison a random enemy for each green gem in the 3x3 area centered on the selected gem.
3.) Transform the selected gem into a gargoyle gem.