No, I still have not found an answer to this issue. I've searched high and low for an answer, queried PHP experts, WordPress folks, searched Google, and so on.
All the suggestions I've received have been WAY off the mark (like 'just include the wp-blog-header.php file, it's easy'). This leads me to believe I'm not explaining the issue properly.
I'm sure the issue has something to do with calling the wp-blog-header.php file from a directory several levels below where it normally gets called from. The obvious answer is to just use an absolute path for the include, and that's the very first thing I tried.
Anyway, I believe this to be the issue because wp-l10n.php gets used in every other WordPress dynamically generated page (it's included in the wp-blog-header.php file) and it works fine.
We can't be the only ones trying to make Gallery and WordPress play together. Heck, Matt (of photomatt fame) uses Gallery himself. But if you look closely, you'll see he does not display any wordpress data (comments, entry titles, etc) on the Gallery-powered pages. All Matt did was make the Gallery pages use the same stylesheet as the WordPress pages. Out of desperation, I emailed Matt to ask if he could assist but got no reply. I'm sure he's probably too busy with serious coding to worry about this, but it's a frustrating problem isn't it? Gallery integrated with Movable Type with no problems.
This has an answer. I just find it strange that no one has come up with it yet.
Do we need to change the includes called from wp-blog-header.php so they are absolute? Should we have another version of the wp-blog-header.php file running from within the Gallery htmlwrap directory? I have no idea.