You make a good point overall, but consider this:
The GP's platform has as one of its main points a moratorium on GMOs:
http://www.gp.org/ecological_sustainability/#esAgriculture
The impact of this, if it came to fruition, on the economy would be nearly unprecedented.
Depending on what definition they use, this result in:
1) Up to 92% of the corn supply
2) Up to 94% of the soy supply
3) Up to 75% of the supply of foodstuffs in general
A moratorium on GMO foods would lead to a catastrophic calamity. Even if they apply the laxest of definitions, even a modest decrease in supply on the teens would be catastrophic, because it unequivocally impacts poor people the most.
It's not so much that the GMO stuff is secondary to their platform and they wouldn't actually go through with it in the event that they somehow win the support to do it: It's that they would even
consider this as something to put on a platform in the first place. It shows a complete lack of preparation and disregard for reality. I would never trust the Green Party to run the country.