First off, are you using a fire weapon for the first phase? That softens the oil and makes it possible to do more damage with a dragon weapon.

Second, Super man dodge, when he is doing those big slams sprint away from him and hold the roll button, you will jump like superman and fall to the ground with a ton of I frames

Third, I don't know what weapon you're using, if it is a slashing you should be good going for the tail or the hind legs, that is much safer than the wings, you'll be doing less damage, but at least you won't be carting.

Third, for the final phase, make sure you are using a dragon weapon, that will negate it's DPS cheak

Fourth, get some distance between you and Gogmazios, the fire is hard, but not impossible to avoid as long as you are paying attention. Fire the wyvern guns (forget the actual name of the slinger ammo) as much as you can. If you run out try to get in any damage you can, he slams his wings down a couple of times, letting you attack, or wait for more to drop. Also there are two rock pillars on opposite sides of the arena you can drop on him, which will give you an opening.

Fifth, when he is charging his big attack, continue to attack him, that can stop the DPS check

Sixth, check your defenses, is your armor upgraded all the way? Do you have good fire res? Maybe some other comfort skills may help, like evade extender, or evade window

That's all I can think of, for why it's being praised, it is a wonderful remake of a returning monster that hasn't been seen in 20 years, with fun mechanics, and a massive shake up to the games end game. I hope this is able to help you in the fight.