Do I need an Xbox Live Gold Subscription to play against other players online?

The Gold subscription is only for playing multiplayer (online games against other players) Some games do not have the single player type game mode and require the Gold subscription to be played.

Yes you will need to get an xbox live subscription to play online multiplayer games. One great thing is if you sign up with a new account you will also be able to get a one month free trial of the service. You also get games with gold every month which
is like getting four free games to play monthly that you get to keep in your gaming library as long as your keep your subscription active. Each month there will be 2 Xbox one games added as well as 2 Xbox 360 games that are backwards compatible. It's a great
deal plus you will be able to get discounts on games each week with your gold membership as part of the Deals with Gold program.

Cheers!

yes, you will need a gold membership to game online.

Yes it is. If your Grand-Daughter wants to play with/against other players online, she will need Live Gold on console anyway.

thanks for your reply - is that normal for a game that you buy ? Seems really ridiculous to buy a game and then pay more to actually access it ?

It depends on the game. Xbox Live Gold is required to play Multiplayer/Online. Overwatch is an online-only game as I don't believe it has a single player component but is just online against other players or co-op with other players.

Battlefront 2 has an online component, hence being asked to buy Live Gold. However, it is not a requirement if the online part isn't being used:

"Online multiplayer on Xbox requires Xbox Live Gold (subscription sold separately)."

I suspect, however, that to over-ride this, the console would need to be put into offline mode. This will need someone else to confirm though as I'm not a console expert