GMail - notify until read? I'm a new Android user with a Motorola X2 running Android 2.3.4 and the GMail app version 2.3.5.2. 

I have a set of labels set up and custom notifications enabled for those that I need to have distinguished. I need a constant alert until the message is marked as read.  

I have been looking at the GMail Label Notifier app but it doesn't appear to have the necessary option. Any suggestions?
The Google's own Gmail app doesn't have this functionality built-in.

Unfortunately with the latest version of the app (v2.3.5.2 as of this writing,) Google have disabled access to Gmail's data by other 3rd-party apps and widgets.  This was most likely done for security purposes.  The side-effect is that none of the 3rd-party apps on the Market that need access to your Gmail accounts (notifiers, widgets, etc.) stopped working.

Your best bet is to find an alternate e-mail app that has such a feature.  I've used K-9 Mail, but I don't remember any settings that would give you non-stop alerts. You could try submitting a feature request on K-9's support site.