The source of their power doesn't matter, a god is a god. Whether they are born with it or discovered it is beside the point. Arguably accumulation of great power over time has a much higher power ceiling than being born a god. But to break down the rest of your statement:
Gwyn's lightning could easily match, if not surpass, Radahn's mastery of gravity magic. Not to mention Radahn's gravity magic is not a good example for your first argument. Radahn wasn't born with an affinity for gravity magic, it was learned at a later date, much like Seath's affinity for and mastery of sorcery. Gwyn could fuck up Malenia too, simply smite that bitch like a hooker in church. She's fast, but she's not faster than lightning.
Malenia's "continent nuke" isn't exactly a good example of a display of power considering she nearly destroyed herself along with everything else. But if you're looking for something in Souls lore with equally destructive potential and innate danger to the one wielding the power, look no further than literally every Lord that has ever linked the flame. Doing so causes destruction on a gargantuan scale.
"In dark souls the bosses accumulated power and influence slowly because everyone was immortal anyway" that's literally the plot of Elden Ring but ok. Nobody and nothing can truly die unless it is slain by Destined Death or Black Flame. You yourself in Elden Ring are an example of an immortal being accumulated power and staking claim as a Lord, that's the entire plot of the game lol
"Even when you fight the big bosses their attacks and moves are pitiful in comparison ER big bosses in terms of power" FromSoft has come a very long way with game design with their games becoming faster, harder, and more complex with each release. Using game mechanics to justify power in lore is not a valid argument in any case.
Examples of DS characters doing similar things to Elden Ring characters include: Gwyn unleashing an unending storm of lightning bolts that slew legions of everlasting dragons, Seath discovering immortality through his mastery of sorcery, Nito having command over death itself and unleashing a miasma that can kill anything, the Witch of Izalith creating life from nothing in the form of great and powerful demons, Artorias stood against the legions of the Abyss singlehandedly, Manus unleashed the dark within humanity and created the all-consuming Abyss, Aldia shed the shackles of fate and freed himself from the cycle of light and dark, Pontiff imprisoned a child of the great lord Gwyn and fed him to Aldrich Devourer of Gods, the Nameless King is a literal god of war capable of commanding storms, Prince Lorian singlehandedly slew the Demon Prince, Filianore isolated an entire civilization from time and preserved it beyond the end of the world, Midir's story is quite similar to that of Artorias albeit with more lasers, Gael is the last living being at the end of time having outlived literally everything else, and the Soul of Cinder is the amalgamation of every being that has ever linked the fire and may just be the most powerful character FromSoft has ever created.
This is far from all, but I think you get my point. Souls characters are immensely powerful, with many of them brushing shoulders with or even surpassing the most powerful among the Elden Ring characters. Please brush up on your lore before you just say things that make no sense.
I've put far too much energy into this discussion but I'm tired of people making this nonsensical argument just because they're turbofans of Elden Ring. Just because you like the game more doesn't mean its characters are all super duper powerful and like totally better than all those other older characters. Gwyn was slaying dragon gods before Elden Ring was even an itch in Miyazaki's ballsack.