Is the phrase “gern geschehen” short for “es hat gern geschehen”?
“Es hat gern geschehen” ; it happened with pleasure?
If not what would a full sentence be like?

Question

Is the phrase “gern geschehen”  short for “es hat gern geschehen”?
“Es hat gern geschehen” ; it happened with pleasure? 
If not what would a full sentence be like?

Answer

gern geschehen is the translation of "you are welcome ". If you want a whole sentence than it would probably be "Das habe ich gerne getan. " loosely translated as "I didn't mind doing that."

Answer

It is like 'you're welcome'.

Answer

So, you can't use the expression (gern geschehen)
Only in this situation:

John: Danke für ihre Hilfe!
Mary: Gern geschehen!