https://idea-hack.com/en/blog/18435/
Solution when Gutenberg’s “embed block”  does not support responsive layout.

Solution when Gutenberg’s “embed block” does not support responsive layout.

This is a way to respond to responsive layout for using the “embed” block with WordPress’s new editor “Gutenberg”.

The reason for this is that CSS code is insufficient.

It is difficult whether it is a bug or a specification, but
Gutenberg ‘s core “embedded” block does not support responsive style by it self. The reason for this is that CSS code is insufficient, and can be solved by loading the following code in style.

css
embed,
iframe,
object {
max-width: 100%; 
}
.wp-block-embed {
    min-width: none;
}

Perhaps Gutenberg’s side did not support it by concern that it would adversely affect the layout of existing themes by adding these styles, and it would cause responsive collapse. so, we encourage you to check backward compatibility as necessary after addition the css code to your theme.