You can do this using a redirect page. Put the following three lines in a text file and name it "index.php":
<?
Header("Location:
http://www.domain.tld/gallery");
?>
FTP that file to your public_html folder, and when someone surfs to
http://www.domain.tld, the "index.php" file will redirect them to the Gallery.
There is a disadvantage; the URL they see in the browser's address bar when they get there will be "http://www.domain.tld/gallery" rather than the shorter version, and if they bookmark it, their bookmark will take them directly to the folder "gallery" rather than to your public_html folder. Those aren't big problems, unless you really want to hide where they are on your site.