wp-replace-figure-with-p-for-gutenberg.php

<?php
add_filter(
	'render_block',
	function ( $block_content, $block ) {
		if ( 'core/image' === $block['blockName'] ) {
			$block_content = str_replace(
				array( '<figure ', '</figure>' ),
				array( '<p ', '</p>' ),
				$block_content
			);

		}

		return $block_content;
	},
	10,
	2
);