Barring the Gate increases the cost of all spells, including friendly.
Many abilities let you cast "a spell with COST 3 or less" for free (Geomancy, Spell Slave, Blood Boon, etc). Barring the Gate doesn't affect what spells can/can't be cast by those abilities. (Infernal Confirmation)
When combined with other abilities that change COST (such as Lamentation) use normal maths: double/halve first, then add/subtract. (Infernal Ruling)
