You could use find and perl (via xargs):
find /target/directory -type f | xargs perl -pi -e 's/stringtoreplace/replacement/g'
or if you are already in the right directory
find . -type f | xargs perl -pi -e 's/stringtoreplace/replacement/g'
and if you only want to replace in, say, html files:
find . -type f -name '*html' | xargs perl -pi -e 's/stringtoreplace/replacement/g'
You don't have to use perl of course, any tool that performs search and replace operations will do, and not doubt there are several available that are less resource hungry than starting perl (which is this case is a sledge-hammer to crack a nut, but it is an example I've had stored for ages and I've no reason to find a more efficient version). My original source for the "trick" is http://oreilly.com/pub/h/73
You can use all the options in which-ever tool you choose, so in this case the full power of Perl's regular expressions. The page linked to above has more examples, including how to search in a case in-sensitive manner.
To undo this, use the opposite command:
gunzip -r ./
Note that the original gzip command will skip over files that already have a .gz suffix, because there's no point in compressing them twice. However, the above gunzip command will decompress such files, because it doesn't know that gzip skipped them.
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You could also try