If the enemy is a level 30 then a level 30 guardian does the damage his weapons says. But if a level 20 attacks a level 26 Minotaur he won't cause as much damage if he had the same gun as the level 30 guy with same upgrades

There's a damage dealt modifier based on mob level to player level. As a 29 against 30 enemies you deal a % less damage and take a % more from them. I have heard it's around 20% difference.

It actually is if an enemy is 1 level higher than you then your damage is cut by 33%. i have tested this and with a 300 attack ice breaker at level 29 precision damage is a little under 9k and at 30 damage is a little more than 12k 2 levels higher than you your damage is about half. 3 is about 75% and 4+ is like little or no damage

Light level makes a difference. It is the truth. Go into a hard raid with 28 ' s and 29's. You will die repeatedly. 29's and 30's, you're fine. Tested and proven by me today ad nausem with raid experienced players for hours. If Atheon or a hobgoblin can one shot you in the raid, do not go. Light matters and of course defense does as well.

I think it's just the correlation between the player level and the mob level, and it is relevant to both offense and defense. ie, use the exact same gun and your lvl 30 character will deal more damage to a lvl 26 mob than your lvl 24 character will do. The big difference between 30 and 24 against a 26 mob makes this easy to illustrate. But the differences, while more subtle, are still there at 28, 29, 30... Since the armor is what dictates your level in this silly universe, that means the better your armor and upgrades, the more damage you can deal, and absorb as well obviously. My lvl 30 character just simply has an easier time doing raids and nightfalls than my lvl 29 character does. He deals more damage and gets quicker kills, while taking less damage. And they both use the same weapons.

I don't think armor helps gun strength much maybe a little but mostly it lets you receive more damage from enemies.