Snippet #011 Custom Gutenberg gradient colors. https://since1979.dev/snippet-011-custom-gutenberg-gradient-colors/

custom-gutenberg-gradients.php

<?php
/**
 * theme_custom_gradients.
 *
 * Add custom gradients to the Gutenberg editor.
 *
 * @see https://since1979.dev/snippet-011-custom-gutenberg-gradient-colors/
 *
 * @uses add_theme_support() https://developer.wordpress.org/reference/functions/add_theme_support/
 * @uses __() https://developer.wordpress.org/reference/functions/__/
 * @uses array() https://www.php.net/manual/en/function.array.php
 */
function theme_custom_gradients()
{
    add_theme_support('editor-gradient-presets', array(
        array(
            'name' => __('Light blue to white', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg, rgba(0,101,155,0.5) 0%, rgba(255,255,255,1) 100%)',
            'slug' => 'light-blue-to-white'
        ),
        array(
            'name' => __('Blue to white', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg, rgba(0,101,155,1) 0%, rgba(255,255,255,1) 100%)',
            'slug' => 'blue-to-white'
        ),
        array(
            'name' => __('Dark blue to white', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg,rgba(29,39,53,1) 0%,rgba(255,255,255,1) 100%)',
            'slug' => 'dark-blue-to-white',
        ),
        array(
            'name' => __('Blue to dark blue', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg, rgba(0,101,155,1) 0%, rgba(29,39,53,1) 100%)',
            'slug' => 'blue-to-dark-blue'
        ),
        array(
            'name' => __('Light blue to black', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg, rgba(0,101,155,0.5) 0%, rgba(0,0,0,1) 100%)',
            'slug' => 'light-blue-to-black'
        ),
        array(
            'name' => __('Blue to block', 'your-textdomain'),
            'gradient' => 'linear-gradient(180deg,rgba(0,101,155,1) 0%,rgba(0,0,0,1) 100%)',
            'slug' => 'blue-to-black',
        ),
    ));
}

/**
 * Hook: after_setup_theme.
 *
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 * @uses after_setup_theme https://developer.wordpress.org/reference/hooks/after_setup_theme/
 */
add_action('after_setup_theme', 'theme_custom_gradients');
?>