The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
Matter of fact, Gimmi is more powerful 1-A+ than you can even imagine.
Meanwhile, SCP doesn't even have anything like a well-defined canon, it's a bunch of random fan fiction and anyone can add more funny feats there with no rules.
Now imagine if I wrote a random fan fiction story also for Gimmi to give him whatever new powers I want him to have.
Would you accept my random fan fiction story as "canon" for Miraculous ?
Obviously not.
Therefore, for unregulated fan fiction nonsense like SCP with no clear canon it's better to match them against Original Characters OCs, who also do not need to follow any rules (like e.g. a consistent narrative) - and not against characters who are from fictions which at least "try" to make sense and to have some sort of consistent story.