Muzzle gauge reads the wear on the muzzle and yes you should pay attention to that number. Here is how it reads.
Carefully, so as not to damage the barrel's rifling or the gauge, insert the the gauge into the muzzle as shown. Read the gauge directly from the side. Observe where the muzzle "falls" on the gauge.
0.0 indicates a .300 diameter bore, that is, .30 caliber, new, perfect.
0.5 is like new, .3005 caliber. Proper re-crowning will perfect the crown.
1.0 is excellent for a used barrel (.301 cal.) Re-crowning should make it like new.
1.5 is good (.3015 cal.). Proper re-crowning should make it excellent.
2.0 is fair (.302 cal.). Proper re-crowning should make it serviceable.
2.5 is poor (.3025 cal.). Proper re-crowning might make it serviceable.
3.0 or more is a "bald tire" and is now a British .303 caliber barrel. Replace barrel
I agree with you on how to use the gage but not with your descripion that a barrel with 2.5 is poor and a 3 is junk
I have personally gaged may unissued in wrap USGI barrrels and many gaged over 2.Very, very rarely do you ever find one that gages 0 new. I would estimate a 1 or more is average
While a barrel that gages a 3 may have had some rds through it , it is not even close to being worn out. There are alot of other factors besides muzzle measurements that can affect accracy in a Garand Stock fit and lock up is a huge factor
CMP Service Grades which are proably their best sellers the muzzle will gauge "3 or less" and the throat erosion will gauge less than 5.
To the OP, You say the seller said it measures more than 3. I would ask for a exact measuremsnt, by his description it could be a 4,5, etc
