During the oppent s turn, he cast an spell, and someone s trying to cast an counter spell. He/she has one of the guild hate land on the field and want to tap it to add an mana to the mana pool and pay that mana to cast the counter spell, does he/she need to tap the guild gate at the beginning of the oppents turn or he/she can just tap the guild gate and get the mana in instant speed and tap it to cast a spell?
Land can be tapped at any time to add mana to a player's mana pool. However, they'll need to spend the mana before the next step or phase, because mana pools empty during those times.