When natsu first used LFD Erza compared the power to when he ate etherion which is when he first used dragon force. Also I don't think it's a stretch to say that LFD is at least a 2x boost since when he went from getting manhandled by hades to wailing on him and overpowering his magic.
Everything is relative. If simply being a dragon slayer can't give them the ability to do significant damage to even low tier dragons then Gray being able to oneshot Mard Geer can't be attributed solely to a devil slaying advantage.
Gray at his strongest equaled a version of natsu that was equal to One of natsu's more prominent power ups. Again Natsu has another power up which is the reason why his max is currently stronger, but if Gray gets another powerup (that's at least 2-3 times his normal power) Then the gap closes.
Also Regular Natsu was frozen for sometime by Invel so i don't see your point. Neither of them are beat spriggans with their base attacks.
LFD was comparable to Dragon Force because Natsu ate all of Laxus' magic power. This is roughly equivalent to all of Jellal's magic power, which Natsu also ate to obtain dragon force before. When Natsu is using LFD with his own magic power, it's much weaker than Dragon Force with his own power. Case in point: DF was better than LFD against Mard Geer.
GMG dragons are much, much, much stronger than the etherious. They were future Rogue's gambit to taking down Acnologia, and future Rogue must have already lived through the Spriggan invasion and all. Plus, you're comparing Gray with Silver's mp against Natsu with his own mp. If we're truly being fair, Gray's with Silver's is actually the counterpart to Natsu with Igneel's power. Both are power-ups that their fathers left them in the form of a tattoo on their arm. It should be pretty obvious why Natsu jumped way ahead of Gray when you look at it like that.
......He copied it by absorbing it. And he got in two hit s while Invel was still surprised at the fact that he was alive. Doesn't mean that Gray would have beat him with just his bare hands lol. and he was bleeding from his abdomen. Just because he chose to ignore it because he knew he had hax doesn't mean it's not damage.
Yeah because Gray's attacks were all nullified by his hax (same way Natsu couldn't hit him until wendy stepped in). That says nothing about his attack power. But we do know that since gray can freeze natsu's fire he could do a similar thing but just freeze madmole for a bit. And take it how you will but the previous time they only one cause of hax and separating their opponents. This time they had counters to the hax and were all in one place.
He never absorbed it. There was no absorbing anywhere in that fight. He copied the Invel's ice with ice-make. Gray was pummeling Invel with his bare hands. He broke Invel's strongest ice with a punch too. He couldn't do this before. Even if you don't think he could have won without copying Invel's ice, there's no denying that Gray was stronger because of the emotional boost he got when Juvia died.
Zero longsword hit Skullion cleanly, and the ice wasn't nullified until after Skullion was encased in ice. That says everything about his attack power.
Gray can probably freeze Madmole, sure but I see Madmole breaking the ice and coming out with virtually no damage. The same way Kyria did.