That's pretty easy.
Remember, I don't know exactly what's the plugin's file name and directory, and the following code was not tested.
Put this to the functions.php
or create a small plugin.
<?php
add_filter( 'option_active_plugins', 'wpse264498_deactivate_fat_gallery' );
function wpse264498_deactivate_fat_gallery($plugins){
// check if you are on the certain page
global $pagenow;
if( $pagenow == 'post-new.php' ) {
// check if it's right CPT
if( isset($_GET['post_type']) && $_GET['post_type'] == 'sln_booking') {
// search the plugin to disable among active plugins
// Warning! Check the plugin directory and name
$key = array_search( 'fat-gallery/fat-gallery.php' , $plugins );
// if found, unset it from the active plugins array
if ( false !== $key ) {
unset( $plugins[$key] );
}
}
}
return $plugins;
}
Also, you can try Plugin Organizer.