Where can you download, modify and share a silicon IP block just like you do, say, with gcc?

Nowhere.

The licenses of mainstream proprietary silicon CAD tools explicitly forbid publishing anything created with them. This is partially due to keep algorithms, such as place-and-route, more secret.

It is like writing a book with a proprietary word processor and then not having permission to publish it for not revealing how the grammar checker works.

Free silicon requires free CAD tools.