What would be the effect of the transfer of glyphosate resistance from a transgenic crop plant to a weed plant?

Ultimately the glyphosate wouldn't have more effect over that type of weed plants and, therefore, the plant would grow freely affecting the crop's yield. This is one of the greatest risks of the transgenic organisms, to transfer genes that are not intended to be passed and lead to a worse case scenario than the one that the inserted gene was meant to resolve. In the case of the glyphosate there even other complications that are real and not just mere presumptions. There is, for example, gut problems in cattle animals that are fed with plants resistant to glyphosate.