They do support some GMO research as part of their commitment to improved agricultural output and food security in the developing world. I suppose it's a case of conflicting priorities. They've been into reducing poverty and hunger longer than they've been into environmentalism.
Even then it's not a straight trade-off. GMO can have harmful consequences, but if it reduces the agricultural footprint that's an environmental benefit.
https://www.globalcitizen.org/en/content/gates-divest-fossil...