add these directives to .htaccess in the root directory of your website
Options +FollowSymLinks
RewriteEngine On
# add trailing slash
RewriteCond %{REQUEST_URI} !(/$|\.)
RewriteRule (.*) %{REQUEST_URI}/ [R=301,L]
# rewrite gallery/ to gallery.php
RewriteCond %{REQUEST_URI} /$
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)/$ $1.php [L]
# redirect example.com/index.php to example.com/
RewriteCond %{REQUEST_URI} index\.php$
RewriteRule ^(.*)index\.php$ /$1/ [R=301,L]