I will answer my own question. I have confirmed with Interactive Brokers support staff that GTC limit orders can be filled outside of RTH.
Interactive Brokers has been a good platform for futures so far, particularly when it comes to pricing. However, it is a complex platform given the flexibility it offers to clients, so a good practice would be to try out on a paper trading account, make some losses in fake money, get familiarised with the platform before getting into real business with real money.