I looked at the site and this is what the web inspector displays:
You can see that within the elementor video widget there is an inner DIV that is wrapping the video:
<div class="lyte-wrapper fourthree" style="width:420px;max-width: 100%;margin:5px;">
It contains an inline style setting the width to 420px.
That wrapper is the cause of the issue. That code is not part of the default Elementor video widget and its most definitely not something related to GeneratePress.
So there must be another plugin or function that is inserting that code.