+SPD or +ATK Gunnthrá

+ATK is the best boon, but -spd hurt....

I would go for the +spd, you loose a little bit of power but you will double more and can score ORKO vs bulky unit with middling speed.
I think the atk boon is not worth having a spd bane 'cause this bane will make you loose a lot of ORKO vs bulky unit.
But if you focus only unit with chilling seal debuff and have hone buff, +atk is the best.

in ur case id keep spd+. while i would prefer atk+ a spd+ gunny can run bladetome to make up for her damage on a emblem team, if ur gunna use her default debuff set then atk+ wouldnt be too harsh as u can nearly 1 hit alot of enemies if they are debuffed properly.

Keep the +Spd. There are so many ways to debuff enemies and for a lot with the new units coming out. Gunnthra can pull of stunts that regular bladetome users have a bit more difficulty doing because they need to worry more about positioning to receive buffs. But Gunnthra has a reverse bladetome effect and applying debuffs is much easier to execute. She can hit triple digit damage against top tier blue units at max merge. I recommend watching Pheonixmaster1's showcase on Spring Catria but it is more about Gunnthra in that video. +Spd at least allows her more doubling potential because she can just get insane amounts of damage with her tome's special ability.