BondoBob....
Imagine you are making replacement aircraft parts that will be sed to keep them flying in a combat zone.
You don't have the plane in question(a chamber) to try your part on.
The mechanic needs a part that fits any aircraft built to specs. Done right,engineering and tolerance study allows a gauge to be built so that if it passes the gauge it should fit any airplane.
Same with guns,reloading,and guages. SAAMI documents the standard dimensions for USA ammo and guns.
Your gauge is the last word that your ammo passes SAAMI dimensions and should run in any SAAMI gun. (for the dimension(s) the gauge is designed to check)
Your gauge is not junk,and IMO,it would be a mistake to retire it to a drawer.
Am I correct,you have more than one .380? What if you buy another? Do you want to discover you loaded 1000 rounds that fit your first gun,but won't run in your new gun?
If the ammo fits your gauge,it shoulf fit any gun made to SAAMI spec.
If your gauge won't accept the ammo,you have a problem to correct. Tilted bullets and distorted cases.
You are taking steps to correct that problem. Great!!
To not fully solve the tilted problem,you have ammo the gauge rejects.
Because your chamber has some diametral tolerance,it accepts some reject,non SAAMI ammo. You can use your chamber to sort reject ammo.
Or you can refine your skill and process to make only good,SAAMI ammo.
The gauge is a good tool to assure your ammo remains quality.
You are new to reloading. You get to decide if you produce quality,or mediocre ammo you have to sort.