The GMF's are produced by inserting the foreign gene to make them disease resistant, increse nutrition and survival etc. They also checked this for toxicity before it has been introduced into market, but the possible risk factor is genetic will vary with indiviudual to individuals, so there is chances of allergies you can seen in this GMF's. And sometimes the gene inserted into the plants may also turns toxic in some GMF's even after years ans years this is called as gene mutation...
The GM food has several types of potential health effects that could result from the insertion of a novel gene into an organism. Health effects include the production of new allergens, increased toxicity, decreased nutrition, and antibiotic resistance, for more details please read in the link:
http://enhs.umn.edu/current/5103/gm/harmful.html
Animal toxicity Many studies used animals models to assess certain GM foods, shown that they may toxically affect several organs and systems. The results of most studies with GM foods indicate that they may cause some common toxic effects such as hepatic, pancreatic, renal, or reproductive effects and may alter the hematological, biochemical, and immunologic parameters.
https://www.ncbi.nlm.nih.gov/pubmed/18989835
The companies which produced GM food focus on the advantages of the techniques to get benefits from their products.
Anyway, to give a good recommendation about this technique required more studies for longe-term effect on the health.