You don't need to consult gnu while playing for money. If you are a person capable of doing a little programing, then you can hook up gnu to play for you. You can even program it not to play too well and include an occasional mistake! It can be done and indeed it often has been done. It is very difficult to detect. However, if you play for money and find that (a) your opponent is playing very well and/or (b) you are losing, then if you keep playing you must be barmy! The overwhelming majority of money players are playing honestly and a lot of them are not very good. The problem however isn't what you lose to the other players, it's what the site rakes from the pot after every game. In the long run it is almost impossible to beat it. It has an effect on your take points as well. If you take a 2 cube at a dollar a point, then on partygammon for example, you will lose $2 if you lose, but only gain $1.76 if you win! Some sites rake more than this.
I have replayed and analysed hundreds of matches from fibs and never seen any evidence that anybody is playing well enough to be suspected of cheating by using a bot.