“Yes, I know that highly targeted searched are much more interesting. But I do maintain, search engine traffic in this sector is very low in term of quantities. 
“
And this is simply objectively not true. “Poker” was in the top ten for all search terms for 2004, for example. That includes “John Kerry” and “Paris Hilton” and on and on. The free search engine traffic in this sector is staggering.
“I have other sites in other sectors and it is relatively “easy” (when you are an experienced webmaster and constantly work on your sites) to score 500-1000 unique visitors a day from Google. I am very far from this with my online gambling sites.”
I’m afraid this is probably more of a comment on your sites or seo abilities in other sectors compared to your sites or seo abilities in this sector. Visitors from 1000 unique search terms a day is doable in this sector.
Gambling is one of the deepest sectors for search engine traffic. People search for all sorts of things you wouldn’t imagine before you got into it. What it doesn’t have is something comparable to “viagra”, an obvious one word flagship term. “Poker” is one for that niche of the bigger sector, but besides that, “gambling” is nowhere near comparable to “viagra” as a flagship term. The gambling sector is about a wide, broad spectrum and volume of terms.