Is it permissible to accept a large sum of money as a gift from a girlfriend, knowing that one is unable to repay it?

Any relationship of love or friendship between a man and a woman who is not his wife must be severed, as it is forbidden by Sharia (definition). The money you took is a debt upon you; if the owner of the money absolves you of it, your responsibility is cleared, otherwise, it must be returned when you are able.