it is utterly a rumor.

There are no solid evidence showing genetically modified plants have negative effects on human and animal health. (Note that logically it doesn't conclude that genetically modified plants have no negative effects.) Also, any food made from genetically modified plants must go thorugh strict processes to show it doesn't product proteins toxic to consumers. Allergy could be a potential issue. It usually requires large scale public health research to verify it. 

As for the example in the post that f2 mice or rats are completely sterile, it is like a joke if you understand some basic biochemistry and molecular biology.

The primary concern is its potential threat on its nearby ecosystem, since the transgenes may escape into local wild species through cros*****reeding.