Jürgen Müller

Just another mask! 
A very lightweight extension for the core/group gutenberg block. 
This adds an option, under the group blocks style tab, to add different top or bottom masks to a group block. 
You will love it. 🙂

Easy Mask

Boring straight lines? Add option to mask the core/group block with waves, slopes, spikes etc. A very lightweight extension for the core group block.

Bogdan Bendziukov

Select a custom mask for your Gutenberg image’s block or add a masked image anywhere via shortcode <code>[wp-image-mask]</code>.
Shortcode attributes:
<ul>
<li><code>src</code> – URL of the image;</li>
<li><code>mask</code> – URL of the mask;</li>
<li><code>fit</code> – how the mask should fit into the image. <code>cover</code>, <code>contain</code> (default <code>contain</code>);</li>
<li><code>alt</code> – alt text for the image.</li>
</ul>

WP Image Mask

This plugin adds the ability to set a mask to Gutenberg's image block or via shortcode [wp-image-mask].

Easy Mask vs WP Image Mask