Gaara's barrier is not prompt, which means he deploys it when its his turn to move.

If you put him as your move 1 ninja and have higher initiative than your opponent it will be used at the beginning of the round, but Gaara can't attacl the same turn the barrier is used.

I'm not sure if using skill breakthrough you can make his barrier a prompt, which would mean he will use it as other ninjas use their mystery, during the move you prompted it. I know that Itachi's barrier can become prompt via skill brakthrough but i'm not sure gor Gaara.