Your critical hits are not affected at all!

If you roll a 20 on an attack roll, meaning the die itself shows a 20, it is always a critical hit, as explained on PHB p. 194:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, [...]

The GWM damage bonus is also not affected, because only the damage dice are doubled. You can still add your brutal critical bonus dice, but the +10 on damage does not change, because it is not a die.

No.

There are two crucial reasons why.

1. The weapon casts the spell after the damage from a successful attack has been dealt. The attack has no effect on the spell other than triggering the spell storing ability.
2. The weapon casts the spell directly on the damaged target with no further attack roll needed. The ability's text says the weapon "immediately cast[s] the spell on that creature," which affects the creature as if the spell was immediately cast on them and therefore needs no attack roll. One could argue a touch spell cast with a spell storing weapon needing an attack roll. However, the rules for touch attacks say that with spells/effects with a range of touch, "the aggressor need only touch a foe for such an attack to take full effect." No attack roll is needed because the weapon is already touching the creature. Without an attack roll, the spell cannot critically strike.

Spell storing casts a spell from an item.

Remember that spell storing uses the rules for casting spells from items as noted in the Using Items section of the Magic Items chapter in the core rulebook. The weapon itself casts this spell when damaging the creature. This is pointed out in the ability's text.

Anytime the weapon strikes a creature and the creature takes damage from it, the weapon can immediately cast the spell on that creature as a free action if the wielder desires. (This special ability is an exception to the general rule that casting a spell from an item takes at least as long as casting that spell normally.) Once the spell has been cast from the weapon..

Many people erroneously compare spell storing to spellstrike. Spell storing does not work the same way as the spellstrike magus class feature. That class feature uses the rules for holding touch spell charges whereas spell storing uses the rules for casting spells from items.