The placement of advertisements is always a matter of which groups are targeted and expected to be found at a place. For this reason, ingame advertisements in a game like this will draw in a certain type of ads as well of course.
With the most prominent and lucrative type of online advertisements (pornography) likely excluded, others will take the niche.
Assuming, that this kind of game is able to attract people who are reward-driven and eager to spend in small transactions, makes betting sites one possible interested partner naturally.
Someone else mentioned to have gotten some social interactive sites as ads; I guess that fits too.
On the other hand, I would not expect commercials for long term saving plans to pop up around here.
I don’t expect potential partners to be standing in a line to the horizon, and typically the payout per ad view is meager enough even without getting picky.
At this point, I got to say, GOW simply has to take what they get (and once they start allowing the above mentioned porn ads, you know they’re getting desperate.)