Is it possible to use an animated GIF as portfolio featured image? If yes, how?

It is possible, but you would need to override the parent theme’s function and customize it in a child theme.

First, you need a new child theme for the job. Please see here on how to create one: https://uxbarn.ticksy.com/article/8649/

Then in the “functions.php” file of the child theme, add this code:

function kathy_get_portfolio_list_image_size( $layout = 'list', $is_full_width = true ) {
    return 'full';
}

The custom code will override the image size of the parent theme and the GIF file's animation should work.