hashirama cannot seal him. pollen has no effect because gaara can spam his flight. plus hes encountered it before and is a sensor.
by the time hashi even gets to his posiition he will sink into a hole of obvilion, buried alive. hes never heard of gaaras abilities and gaara would sense him coming. this distance is nearly a football field. so gaara can easily grind up the stones beneath hashiramas feet and bury him after taking flight to avoid attacks. if hashi escapes, gaara drops a pyramid on him. he wins both scenarios.