The Dutchman was charged with making comments that "alleged and/or implied bias on the part of the match referee, and/or brought the game into disrepute" after the Premier League club's 0-0 draw at the Abbey Stadium.
Following the stalemate - which led to a reply that Manchester United won 3-0 - Van Gaal told reporters: "Every aspect of the match is against us - the pitch, the referee. Everything that you can think about this is against you."
Van Gaal decided to appeal the decision, with the FA confirming on Wednesday that he had been warned.
An FA statement read: "Following an Independent Regulatory Commission hearing on Wednesday 18 February 2015, Louis van Gaal has been warned as to his future conduct after he was found to have breached FA Rules in relation to media comments."
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