You do realize, right, that every GMO is required to undergo years of testing?
True but the testing they undergo is less rigorous than drugs and yet every so often a new drug has to be recalled because of either rare side effects or long term effects that were not known at the time of release. That is not a reason to ban GMO since, as with drugs, the benefits can outweigh the risks. However we would never dream of giving someone a new drug without telling them what they were taking so why should it be ok to let people eat GMO without telling them?
If GMO labelling were mandatory then companies would be forced to pass the benefits onto consumers: if GMO strawberries are cheaper to grow then they should be cheaper in the shops. This combined with an education campaign would mean that people would see and understand the benefits of GMO and so be more supportive of it. By hiding it the corporations can pocket the savings instead of us and they don't have to bother educating anyone which perpetuates the resistance to the technology.