Block in widget

Sure, in a text widget just wrap the shortcode in the PHP function
<?php echo gcb(x);?>
where x is the content block ID.

You may need to install a plugin that allows you to use php in widgets.

Could you add a widget with the plugin that accepts short codes? So no PHP-accepting widgets are needed?
Thanks!

Not exactly sure what you mean but you could try it and see if it works?

this could also be done by just using the shortcode itself in the text.
Eg: [contentblock id=xx] inside a text widget. The text widget evaluates shortcodes. If you have another widget, then you need to make sure it evaluates shortcodes in its content processing function.

If you provide an example, I could assist you further