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What does the wiring diagram/instructions indicate?
If power needed for the gas boiler is a 15-amp circuit, then you could run 14-2w/g to power it with a 15 amp breaker for your load center.
If power requirement is a 20-amp circuit, then you would need the 12-2 w/g and a dedicated 20-amp breaker in your load center. Frankly, most of the boilers I've done only require a 15-amp circuit so I would assume that yours would too, but you need to check the specs.
If you want, and it needs a 15-amp circuit, you could certainly use the 12-2, but it is stiffer to work with, especially for those who don't do this every day.
In other words, you can use higher rated wire, switch and breaker, for the circuit, but you can't use lower-rated wire, switch and breaker. Hope this helps.