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.
They first fought in Thor #160 - #161
Basically the first fight, they fought evenly but Galactus was winning before Thor came in. You can read here: https://readcomiconline.li/Comic/Thor-1966/Issue-161?id=7711#1
Second time they fought was in Thor #226
Ego was winning and Galactus needed Thor and Hercules' help
Thor even said that Galactus couldn't defeat Ego on his own that same issue
So they are in the same ballpark