The Great Weapon Master feat (PHB, p. 167) says:
- Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.
The Spiritual Weapon spell description reads:
You create a floating, spectral weapon within range that lasts for the duration or until you cast this spell again. When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon. [...] The weapon can take whatever form you choose.
Does the chosen form determine the stats of the weapon (and therefore grant the Heavy attribute that allows the listed GWM benefit to be used), or is it just a cosmetic decision?