I need help with following:

Remove that and try this instead:

@media (max-width: 768px) {
    .entry-header {
        text-align: center;
        border-bottom: 1px solid #000;
    }
}

It is making my title and meta aligned left but I want it in the center only on mobile view.

Try the edited code:
https://generatepress.com/forums/topic/i-need-help-with-following/page/2/#post-910629

It is working...
As I said, I want to control line length also. So Please help...

The width cannot be controlled in border property unfortunately.

Try creating a new hook:
https://docs.generatepress.com/article/hooks-element-overview/

With this as the content:
<div class="border"></div>

And select after_entry_header.

Then display for all posts.

Do that first and let me have another look :)