Using gtags with Ruby and C code

Hi,

I'm using gtags to navigate Ruby code using the instructions at this
page http://imaginateaqui.net/blog/2008/10/using-rtags-ang-gtags-for-coding-ruby/.
 The code I'm working with also has some C code extensions. I would
like to use the same GTAGS db for both. I did a first pass with rtags
per the instructions in the page above. Then I do export
GTAGSLABEL=default and wanted to update the same db with just the C
file tags using the default parser. But that does not work. How can I
do that? Essentially I'm trying to add tags from two different labels
to the same db.

thanks
sundar