how do you add hide-on-mobile class to an element?

Hi there,

Are you talking about a block element or a hook element?

Let me know :)

Yes. I am talking about a block element. Check my site. Its homepage is good on desktop. But not on mobile. So, I want to make a different element for mobile. And I will disable the first one for the mobile and the second one for pc. Is it ok? and how to do it?

You can add the class to the Additional CSS class(es) field for the main container:
https://docs.generateblocks.com/article/container-overview/#additional-css-classes

After that what will I do?

That should be all you need to do.