Gut, True Soul Zealot

Halsin, Emerald Archdruid's activated ability overwrites any previous effects that set the creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Halsin's ability resolves will overwrite this effect.

Effects that modify a creature's power and/or toughness, such as the one created by Titanic Growth, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch power and toughness.

Once the game begins, your two commanders are tracked separately. If you cast one, you won’t have to pay an additional {2} the first time you cast the other. A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background won’t usually be a creature anyway).

Gut doesn't have to be among the attacking creatures.

If you control a Background that grants an ability to commander creatures you own, and you own more than one commander creature, each of them will have that ability.

If your commander loses the choose a Background ability or stops being a Background during the game, as appropriate, it is still your commander.