Gaia doesn't control the offer, you need to report to the offer provider if you don't get your cash (there should be a link on the offer pages with each provider where you can check the status of offers)
While some offers are fake, other could just not have completed properly for a number of reasons.
If contacting the offer provider doesn't help, there is a thread in the sponsor forum where you can report them.
I don't usually have problems getting cash from completed surveys..... It is the qualifying for the surveys that I find hard and why I usually avoid them.
There is lots of helpful information in the sponsor forum on what offers are good and which to avoid.
Personally I just do the cash tree and video offers, never had a problem with them.... I only do surveys when I need a high payout offer to get the bonus item.