Unless your gunning guys on tour or extremely accomplished Div 1 type guys with live arms, 97 mph is lightning fast. Assuming the serve is somewhat accurate and repeatable, it's an excellent serve; congrats! 120 mph is simply another dimension.
And fwiw, of course the position of the gun matters; it's measuring bounced back waves and the closer in line with moving object, the more accurate. (Small incident angles are fine but they won't measure from the side.)
I have a pocket radar; it's amusing to play around with. I find the very experienced (teaching pros/ex-div 1 types) are impressed with its accuracy and size; the young kids and rec players always shake their heads and comment that it has to be reading too low. lol.