y, you need to first authorize MT for your account. Gain doesn't have us on their portal yet, so you have to email them to do that (very quick turnaround). Just email
Send an email to gfsupport@gaincapital.com – ask them to enable particular accounts (give them the usernames) for both UUIDs of Medved Trader. Do tell them whether the accounts are live, simulated or developer. NOTE: unlike most programs, MT has TWO UUIDS with Gain – one for quotes and one for trading. They need to enable for both, so it is best to make sure to explicitly specify.