Adding SilverStripe header to Gallery. I'm assuming you're using Gallery v2.x. Also, I forgot to mention that the header isn't going to be dynamic. If you add/change any of the header menu picks you'll have to change them in Gallery too.
In my case I'm using the matrix theme in gallery. go to your photogallery/themes/matrix/templates directory. Create a sub directory called local and copy the theme.tpl file to the local directory. We don't edit the default template, but this copy takes precedence. Edit the newly copied local/theme.tpl file.
We need to insert a few things from your silverstripe site. We need to insert the stylesheet info and the basic html that builds the header. Get that by doing a view source on your site in a browser. You'll see there is already a stylesheet in the theme.tpl just copy and paste your stylesheet line in after it. You may have to edit it (I did to give it the full path to it).
Then I copied from the viewed source from my "<div id="BgContainer">" to the closing series of </div> for the navcontainer.
Here is what the top portion of my theme.tpl looks like
{*
* $Revision: 16727 $
* Read this before changing templates! http://codex.gallery2.org/Gallery2:Editing_Templates
*}
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="{g->language}" xmlns="http://www.w3.org/1999/xhtml">
<head>
{* Let Gallery print out anything it wants to put into the <head> element *}
{g->head}
{* If Gallery doesn't provide a header, we use the album/photo title (or filename) *}
{if empty($head.title)}
<title>{$theme.item.title|markup:strip|default:$theme.item.pathComponent}</title>
{/if}
{* Include this theme's style sheet *}
<link rel="stylesheet" type="text/css" href="{g->theme url="theme.css"}"/>
<link rel="stylesheet" type="text/css" href="http://actionsuperstars.com/themes/actionsuperstars/css/layout.css" >
</head>
<body class="gallery">
the
<div id="Container">
<div id="Header">
<h1></h1>
<h2></h2>
</div>
<div id="LinkBar">
<div id="Navcontainer">
<ul id="Navlist">
<li><a href="/home/" title="Go to the Home page" class="current">Home</a></li>
<li><a href="/photo-usage/" title="Go to the Photo Usage page" class="link">Photo Usage</a></li>
<li><a href="/contact/" title="Go to the Contact page" class="link">Contact</a></li>
<li><a href="/keeping-your-memories/" title="Go to the Keeping your Memories page" class="link">Keeping your Memories</a></li>
</ul>
</div>
</div>
</div>
<div {g->mainDivAttributes}>
{*
* Some module views (eg slideshow) want the full screen. So for those, we don't draw
* a header, footer, navbar, etc. Those views are responsible for drawing everything.
*}
{if $theme.useFullScreen
Hope this helps. You can view my if it helps by going to:
http://actionsuperstars.com silverstripe
http://actionsuperstars.com/actionsuperstars photos using Gallery
Good luck!
Forgot to mention that you'll have to log in to galley, go to site admin, maintenance, then delete template cache to see the changes.