It's only possible using a mod rewrite (on any system); but the software could easily be modified to handle it. That kind of URL would have to be rewritten to execute a script with the user as a parameter and the showgallery.php script could be modified to take a username as an option to search for. About 8 lines of code with a line of rewrite.
Keep in mind, this may not work in all systems where using use non-alpha/numeric names like "Tom & Jerry":
http://www.yoursite.com/Tom & Jerry
would not be a valid URL.
Please do not PM me for support or sales questions. Thank you for your understanding.