Like this
find . -size +100M -exec gzip {} \;
If you are thinking of running it regularly, you may wish to exclude already gzipped files like this
find . ! -name "*.gz" -size +100M -exec gzip {} \;
And if you have lots of big files and (say) a quad core CPU, you could do 4 at a time like this
find . -size +100M | xargs -n 1 -P 4 gzip