Where I come from, Gack always refered to the residue left over from smoking bongs or buckets.
Gack was scraped out of the cone or the bucket 'head' and ususally chucked in an ashtray. However, those on the dole and rather skanky used to smoke this gack by wiping it on a rizla.
They were known as gack heads. Nasty.
"Duuuude, take a lug on this..."
"Err, no fanx - it's full of gack!"