Yes, I think what you may be looking for is a Login postHook. If you name the users' galleries based on their username or ID (I would do this in the alias), you can use $modx->sendRedirect() in the postHook to send them to their gallery's main page. That way you don't need &redirectToPrior or &loginResourceID.
In the gallery template, you can use a snippet to redirect anyone who is not logged in or who is not on his or her own page to the Login page, also using $modx->sendRedirect().
With those two options, you don't need to use the MODX permissions system.
The snippet on the gallery pages would look something like this (untested):
<?php
$loginPageId = 12; /* set to ID of Login Page */
$url = $modx->makeUrl($loginPageId, "", "", "full");
if ($modx->userHasSessionContext('web')
&& $modx->user->get('id') == $modx->resource->get('alias') ) {
return '';
}
$modx->sendRedirect($url);
I used the ID because the username might contain characters that would be illegal for a URL.