@Djimbalimbl we’ll get your account sorted out for you.
Usually when a player hits this gold cap it’s because they’ve been cheating however, I can see you’re not cheating.
This issue is near the top of the to fix list for the team right now. Before it wasn’t so much of a problem - we hadn’t received reports about it from legitimate players until recently - but as the game is getting older and players have had more time to save their gold to such high amounts we want to do something about it in case others save their gold like you.
I know it’s irrelevant to getting your issue solved but while we have you here could I ask why you’ve been saving your Gold past 2 Billion instead of spending it? I will pass on your answers to the designers as well.
EDIT:
Also just confirming what some people have already said in this thread, the limit is imposed by the way these numbers are stored with code, it’s not an arbitrary cap we’ve placed, but there are things we can do to get around this which we are working on.
Also also, please stop discussing bans on the forum. It is possible for some players to have this bug and not cheat, it is also possible for some players to cheat to get 2billion gold. This gold cap error and the cheating are both completely separate and each is being treated as such.
ie. the error message is caused by hitting the gold cap - how you hit the gold cap doesn’t matter, the gold cap is an issue in and of itself.
It has nothing to do with the hack detection, it’s just you’re more likely to hit this cap if you’ve been cheating than most players are to hit the cap legitimately - but it is perfectly possible to hit it legitimately it’s just far less likely as most players who play this much are sinking gold into ie. Legendary Tasks so rarely hit the cap - until recently as there’s faster ways to earn gold now and the game is almost 8(?) years old so everyone has had much more time to save.
Also just confirming the OP isn’t banned for cheating - they are being blocked by the error message though which isn’t a ban it’s the bug 
Hopefully this helps explain this generically - we’ll have to delete further posts about bans here so please don’t.