I had left the Google Product Category Default setting in the configuration blank because our stuff would fall under 5 different categories:
RescoCCC is correct. If the configuration is blank here, then no Google Product Category will show on the feed.

Why not enter the category as:

Vehicles & Parts > Land Vehicle Accessories > Motorcycle Accessories>Motorcycle Protective Gear >

or

Apparel & Accessories > Clothing > Activewear > Motorcycle Protective Clothing >

Then add the required Google attributes. If the attributes don't apply, put in something like "mixed" or "All". You can then hide all the attributes with the below steps. Won't show in cart, but will be added to the feed. Also permitted to use just:

Vehicles & Parts > Land Vehicle Accessories > Motorcycle Accessories>

Don't have to take the category all the way to the end (ie. Motorcycle Helmets, etc.)

if that is the case, it may be easier to add this:

http://www.zen-cart.com/forum/showthread.php?t=83814

It's only one file. On that one file, you can remove the below code and the attributes will not display on the products page. We've tested it out and the attributes do get included on the feed. Just don't show on the page.

<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr valign="top">
<td width="50%"><?php echo $left_col; ?></td>
<td width="50%"><?php echo $right_col; ?></td>
</tr>
</table>

Our first goal was to get the attributes listed in columns. But since we didn't have "variations", we didn't need the attributes to show on the page. So I removed the tables and it works fine.