Given that you can't "soundly test" the long-term effects of GMOs on humans without first exposing humans to GMOs for a long time (decades, even generations) I don't see how such a label would be of any use whatsoever to inform the purchasing decisions of the first few million/billion
willing volunteers unwitting victims guinea pigs.
Given that a few million/billion people might be seriously ill or dead by the time the "sound testing" for a particular GMO was completed, and the liability bill could financially ruin any corporation, I don't see why anyone would run the risk of creating or backing such a label.
Even if such "sound testing" did occur, do you seriously believe that negative results would be made public? I don't. I'd expect the shredders to be running hot whenever testing data made its way back to head office.
"Grass-fed Beef" has been around for thousands of years, and has an established track-record of safety. Monsanto's "Moo-231987422/A" has been around for a few months, and is made by perhaps the most evil company in the world — with a track record of making nasty things that do bad stuff to animals, plants, and humans. Most rational people — interested in self-preservation — will opt for Grass-fed Beef on their plate.
Profit-driven corporations are immoral or, at the very least, amoral. Time and time again they have shown this to be true. The public don't trust them because they have a track record of deception, lies, and generally not caring about their customers. If the public can't trust the companies, they can't trust the products they make, either. The 99% of corporations that are untrustworthy have ruined it for the 1% of corporations that are actually trying to do the right thing. It's not the consumer's fault — it's corporate culture driven by greed that's at fault; it's capitalism.
If any certification body ever comes into existence to perform the function you desire then I think you'll find it is funded by the very corporations that would profit from its labelling. A mere puppet that can falsely promote customer confidence and be quickly liquidated when the class action lawsuits come rolling in.
At the very heart of the problem, though, is the fact that you can't label a GMO as 'safe' without exposing people (and their unborn children) to unknown levels of harm first. It's a Catch-22. You need willing tests subjects to verify that something is safe, but those test subjects want to know that it is safe before they're willing to eat it.
I don't know if it's even possible to solve that problem.