equivalence of gzipped and non-gzipped files?

[Dalton Harvie]

I'm trying to get a backup system going using rsync --- bit of a newbie.  
Just wondering if there is a way I can get rsync to recognise that a 
non-gzipped file in one directory tree is equivalent to the same gzipped file 
in the (say backup) tree, or vice versa.  Could this be done with the 
include/exclude features?  I note that gzip doesn't alter the timestamp on a 
file when it zips it and I'd be happy enough to use this only to verify when 
files were last altered.

Thanks.