I was able to resolve this by making the following modifications to the Gallery snippet!! I'd suggest, however, that you create a new snippet called GalleryPage or something. Make the following changes in the new snippet so you have the ability still to use Gallery both with and without getPage.
I'm using Gallery1.5.0-pl.
After...
$start = $modx->getOption('start',$scriptProperties,0);
on line 39, add...
After...
$count = $modx->getCount('galItem',$c);
on what's now line 111, add...
$modx->toPlaceholders(array("total" => $count));
What this does, is instructs the "start" parameter in Gallery to read the "offset" parameter of getPage. Also, it sets the placeholder "total" with the total number of "rows" in the response. These are requirements of getPage.