They should be GTC, but there was a bug in the TWS plugin a number of months ago where the GTC was not preserved across orders, but that's been fixed.
I've got the following code to submit a limit order. I've set the BarsValid = -1 but they're still being
sent to TWS as DAY orders - how do i get GTC orders
Dim ordSettings As New OrderSettings()
ordSettings.TransactionType = TransactionType.Sell
ordSettings.BarsValid = -1
ordSettings.OrderType = OrderType.Limit
ordSettings.LimitPrice = position.EntryPrice.SymbolPrice+lim
ordSettings.Size = TradeSize/2
ordSettings.Description = "LT-1"