I used to get free gems (over 20 per day) from watching videos through the Fyber offers. However, it's been well over a year now and I've not received a single video offer. I have seen others post that they earned gems through the offers to download and play games through Fyber also but cannot personally attest to that.
Personally, these are the only two offers I would entertain to complete for free gems. Not only do the offers have disqualifying fine print that is ambiguous, the single time I tried to complete a survey for free gems it was a ridiculous hoop jumping seven part survey. I didn't finish it but I still got spam out the wazoo and robo calls for months after!
As for the gem reward itself: It is my understanding that the gem rewards do not come through immediately. You should be able to see your reward status in the Fyber drop down, some games have a Status tab under the left hand menu while in others you have to look under the Support tab. If your status is anything other than completed then it's just a waiting game until the offer completes in their system.
I couldn't find anything on this in the CS thread, but I did find an informative mod post in the DS thread: