IsTradeAllowed not returning what I would expect

Please see the script below:

void OnStart()
{
   Alert(IsTradeAllowed()); //alert 1
   Alert(IsTradeAllowed(NULL, TimeGMT())); //alert 2
   Alert(IsTradeAllowed(Symbol(), TimeGMT())); //alert 3
   Alert(IsTradeAllowed("GBPUSD", TimeGMT())); //alert 4
}

This returns:

true //for alert 1
true //for alert 2
false //for alert 3
false //for alert 4

As alert 2 returns: true, then I would expect alert 3 and alert 4 to return true.

I have tried running the code at multiple times of the day on weekdays. The code returns the same result at weekends. I have also tried putting the code in a script and an EA. Every time I get the same result. Is there an explanation for this? I have tried what is suggested here: https://www.mql5.com/en/docs/runtime/tradepermission

Symbol() returns: "GBPUSD". Each alert should return true in my mind, however this does not appear to be the case here. Incidentally I have notices that Symbol() returns the symbol at the top of the demo account watchlist if the script is run inside MetaEditor, however it returns the symbol displayed on the chart if run inside the demo account.

The broker is Oanda.

Update 04/03/21 at 19:55

I have now discovered that if I right click on the Market Watch and select: show all, then more symbols appear. I can then see that some symbols are greyed out and some symbols are not. The symbols that are not greyed out e.g. USDGBP-g return what I would expect when running the program above i.e. alert 1-alert 4 =true. The symbols that are greyed out e.g. USDGBP returns true; true; false; false in the program above. I now have two questions:

1. Why does: IsTradeAllowed(NULL, TimeGMT()); //alert 2 return true for symbols that are greyed out?

2. What does -g mean in GBPUSD-g?