Turnstiles open at 1.30pm. I've no idea how many tickets are available, but we don't always reach full capacity on the day even after all the online tickets have gone. I guess you'll still be able to get in by 2.30pm, because I often see people paying on the gate at that time. I can't believe any game would sell out much earlier than that, but it can't be guaranteed and no one can give an accurate answer to your question. You really need to weigh up how much you want to see the game and how inconvenient it will be if you make a wasted journey, then just get there as early as it suits you.