Custom template for Single post

You may not need a custom template.

You would need to add the category class to the single post though with a function like this:

add_filter('body_class','add_category_to_single');
function add_category_to_single($classes, $class) {
    if (is_single() ) {
        global $post;
        foreach((get_the_category($post->ID)) as $category) {
            // add category slug to the $classes array
           $classes[] = $category->category_nicename;
        }
    }
    // return the $classes array
    return $classes;
}

Adding PHP: https://generatepress.com/knowledgebase/adding-php-functions/

Then you could target that featured image like this:

.your-category-name .page-header-image-single {
    max-width: 300px;
    float: right;
}

Adding CSS: https://generatepress.com/knowledgebase/adding-css/

That should get you started :)

Uh WOW, that worked perfectly well...

I might need a little bit of help, if you can.

How can I put the image below the title, so that way the title size will be 100% of the content area?

You should be able to do that in "Customize > Layout > Page Header" by changing "Single Post Page Header" to "Below title".

Let me know :)

The thing is I only need that image position in one unique category posts. On the other categories, image's posts should be above any element, including post title.

That's a bit harder, but you should be able to do this:

add_action( 'after_setup_theme','tu_category_specific_page_header' );
function tu_category_specific_page_header() {
    // Only do this if we're in category IDs 1, 2 and 3
    if ( in_category( array( 1,2,3 ) ) ) {
        remove_action('generate_after_header','generate_page_header_single_above', 10);
        add_action('generate_after_entry_header','generate_page_header_single_below_title', 10);
    }
}

I guess this is solved with the new Elements module on GP, right?

Which part, specifically?

Hello Tom. I'm just gonna close this thread because I can't even remember what site I was talking about here (and if is still on my hands, which I don't think so) and, honestly, I don't like to see the topic open. You know, OCD issues... n_n