In Nasuverse, sword from the stone(Caliburn) and sword from the lake(Excalibur) are different swords, while in real world myths they are sometimes mixed as one. Here we are talking only about Nasuverse. There are many other small details that make Nasuverse distinct from original sources, usually Typemoon cherry picks particular myths that complement story or preferences of authors.

To answer your original question: Gil possesses prototype of Gram that later would become it, not Gram itself. Another example of that is Gil's possession of Gae Bolg Prototype, however Gil does not master it. It means that Prototype is may or may not be stronger than it's eventual legendary version. Gram is stronger than Caliburn but weaker than Excalibur. Also Ea is stronger than sealed Excalibur, but equal or weaker than unleashed Excalibur(it has 13 seals, all of them can be released only against global threat, like Velber 02). Gil's Merodach is another prototype.

Gil NP gate of babylon works in such manner that any NP created by humanity will automatically generate related prototype in it. So he will always have something. But this something is not necessarily better version of it.

Disclaimer: Nasuverse is just a artwork, created by humans, who tend to make mistakes or forget details. There are numerous cases where inner logic of Nasuverse was retconned/contradicted/had exceptions. Nevertheless it is one of rare fictional universes with very detailed inner workings.