(Warning:this is another
GM node.
Ignore as necessary)
A modified crop plants (or other GMOs) are substantially equivalent if there is no difference in toxicity from its traditional counterpart. That is to say, if you make a fish/melon it must be no more toxic than 'normal' melons.
There are those who consider this a rubber stamp to avoid full toxicological tests. However, the tests are (presumably) as substantial as tests for normal crops - more severe testing just because it is a GM crop is prejudicial. A GMO stands or falls alone; there is no reason to believe that all GM foods are intrinsically toxic. There may well be toxic GMOs - and these should be tested in the same way as normal crossbreeds.