Why wasn't the word guuguu in my Japanese dictionary?

When I went to 123japanese.com I was told that guuguu means snoring and I typed in snoring and it turns out it didn't exist. So what is going on? Did I have a bad Japanese dictionary? This site tells me it means fast asleep so please check to see if I was fairly mistaken and I usually think that these onomatopoeia words would be written in Katakana and it was never found. How is guuguu written by the way so I can see it?