Gravity Axe - Grarl
Group: Spell Card
Type: Spell
Icon:Equip
A monster equipped with this card increases it ATK by 500 points. As long as this card remains face-up on the field, monsters on your opponent's side of the field cannot change their battle position.

so does this mean that if he sets a monster he cant flip it?

Yes that is correct, the only way your opponent can change the position of a monster (any position), would be via another effect.

Yep..usually when cards merely say "cannot change their Battle Position" it usually only applies to manual battle position changes (Flip Summoning, manually switching from attack to defense), but not card effects (Book of Taiyou, Book of Moon, Tsukuyomi, Zero Gravity, etc).

If it were to apply to Battle Position changing in general, it'd probably add in "under any conditions" like Mask of Restrict.