As has been noted, the question is irrelevant because there is no Somatic Component. If there was, however, I would argue that you would need Warcaster to use a two handed weapon. While I'm generally not too fiddly about such things, you have to make an attack as part of the spell, which would be when you are using a free hand for the somatic component. Obviously the designers were aware of this and removed the Somatic Component (or as I like to think of it, the attack IS the Somatic Component!) for both Booming Blade and Green Flame Blade.