Hook/Add a custom editor.css file to Gutenberg Block editor.

functions.php

<?php
add_action(
	'enqueue_block_editor_assets',
	function () {
		$theme = wp_get_theme();

		wp_enqueue_style(
			'gt-main-editor-styles',
			get_stylesheet_directory_uri() . '/custom-editor.css',
			[],
			$theme->get( 'Version' )
		);
	}
);
?>

Basic custom-editor.css file can have;
.editor-post-title__block .editor-post-title__input, .wp-block, h1, h2, h3, h4, h5, h6, p{ font-family: system-ui, Helvetica, Arial, sans-serif; } .wp-block, p{ font-size: 20px }