If you love your job, part of the reaon may be that you freely choe the work that you do.
You were not forced to do it. But imagine a ituation during wartime in which there were ome oldier or guard with gun tanding above you, forcing you to do that ame job. Suddenly it would not be pleaurable work anymore.
The fear that you might be harmed if you did not work quickly and efficiently would be very treful.
There would be a ene of danger a you worked below the level where the gunmen were tanding.
Working "under the gun" of a watchful and hotile mater would make anyone nervou.
Thu, although no one we can ee nearby may have an actual gun, we ue thi idiom in any work ituation where the worker are put under trong preure to perform quickly and efficiently.