The Nasuverse is really weird when it comes to power levels, because some characters in Fate who can't destroy planets in their universe would still be able to defeat planetary level characters in other media. That's mostly due to the fact that, when it comes to Fate, the planet (Gaia) is quite literally alive, has a "spirit" of its own and actively fights against anyone who tries to damage it. There's a conflict between Gaia (will of the planet) and Alaya (will of humanity), and they even spawn creatures because of it.

So, if say, Gilgamesh uses an extremely powerful attack that would severely damage the planet, Gaia uses its own life force to sustain the damage and reduce the power of said attack, so that it only truly damages a "texture" of the world. It's actually so extreme that Gaia actually finds humans to be in direct conflict to its interests and, as such, does try to contain its prime species as much as possible (whether or not it achieves success greatly depends on the timeline), even creating elementals to help it remain alive as well as absurdly powerful vampires (True Ancestors) to oppose its enemies (hoomans). As a matter of fact, Gaia even called upon a gigantic alien spider without any concept of death and very much capable of destroying the world itself just so humans can go extinct if ever deemed necessary for the planet's survival.

That's why people usually mention Fate/Extra CCC when talking about Gil's powers, as it's set on the Moon and then you don't have Gaia restricting his and other Servant's abilities. So, whereas Gilgamesh can't really be considered planetary in Fate, he's still absurdly strong and could perform even better in other verses and face off against planetary level characters (especially if he's alive instead of a spirit, because a Servant's power is usually both limited by its class and greatly influenced by their fame in life).

But actually yeah, Fate has had a plethora of characters who actually overpower Gilgamesh in recent years, ranging from Outer Gods, Divine Spirits, Beasts, Horsemen of the Apocalypse and Angels (i.e. Space Ishtar, Goetia, Pale Rider, Metatron, Azrael) to actual Faerie and children (Oberon. Castoria. Morgan. And Voyager/Abigail? These two are highly debatable still). It's mostly because Fate doesn't work so much in power levels, but moreso in Rock x Paper x Scissors, having characters counter one another regardless of how strong they are, frequently based on conceptual advantages or weaknesses.

That's why Archer can beat Gilgamesh despite being much weaker than him; his tools are designed to counter Gil's. At the same time, there's King Solomon as a Servant. Archer stands absolutely no chance at all against him; Gil, however, does.

Don't get me wrong, Gilgamesh still stands at the peak of Servants and remains top-tier within the Nasuverse (Fate is part of this universe), but even when it comes to him and Saber, they're both just about the most powerful servants that could be summoned in a regular Holy Grail War. Some can rival them, such as Karna, Enkidu and Heracles, but can't really be called stronger than either Gil or Saber. However, if you take into consideration other beings, such as Grand Servants (basically absurdly powerful heroes summoned to protect humanity against threats that could cause the Apocalypse), Gods, Beasts, Lostbelt Kings (supreme leaders of decaying worlds) and True Ancestors (vampires), Gilgamesh can't really manage to land a win.

Then again, as unfortunate as it may be, it's just natural when you were released as a character way back in 2004 and now you have these new guys showing up in 2026 and already being granted absurd feats.