The phrase "not a gamble" is correct and can be used in written English.
For example, you could say, "I believe our decision to invest in this project is not a gamble."
That's not a gamble we should be taking".
Voting for him is not a gamble on unknown potential.
To me, that is not a gamble, that is a bad idea.
Note that this strategy is not a gamble on the direction of the overall market.
NUSSBAUM -- They're so cheap to make; it's not a gamble, even if it flops.
If we take a decision like that, we know it's not a gamble.
A large red sign on a back wall says, "Attention Customers: Please be advised this is not a gambling establishment".
"I'm not a gambling man but if I had to bet a fiver, I'd say Ison will survive,' said Brown.
It was Michael Jordan's father, after all, who explained that his son's heavy wagering was not a gambling problem but a competition problem.
I'm not a gambling man, but if I was, I'd probably be deeper in penury than I am already.
