i have a working algo and i know have to use the C# method but I'm a little confused about the brokerage.

I have this is my algo:

SetBrokerageModel(Brokerages.BrokerageName.GDAX, AccountType.Cash);

I guess GDAX is now coinbase pro and i have an account but I don't see any option to fund it with USD, am i supposed to fund the account with USDT or how does that work?

I.E. what is this "your brokerage cash"

Setting Cash

In backtests you can set your starting capital using the SetCash(decimal cash) method. In live trading this is ignored and your brokerage cash is used instead. In paper trading we set the cash to a fictional $100,000 USD.