You have to design a payload that can survive a nuclear explosion imparting the entire launch energy in an instant.

Sure, maybe you can launch a solid 100T block of steel. What do you do with that once it's in orbit? There's no space lathe to turn it into anything useful. Can you actually design a survivable vehicle to carry any other payload?

Assuming we have some reason to launch 100T blocks of steel into space, where are you sourcing them from? How can you transport them to the site? How are you going to load them into the gun? Does anyone need 100T blocks of steel in space right now?

How long do you think your barrel is going to survive repeated nuclear blasts for launch? How are you going to replace your barrel when it needs to be changed?

How much is it going to cost to build your gun? How many decades of launching 100T steel blocks into orbit, for which we have no orbital manufacturing capability whatsoever, will it take to make your money back?