The phrase "gat someone on horn" is a colloquial expression used to refer to the act of placing a phone call to someone. When looking for synonyms, there are a few different options depending on the situation. A more formal term would be to "initiate a phone call," "make a call" or "place a phone call." A more informal or slang option could be to "give someone a ring," "buzz someone," "hit someone up," "call someone up," or "ring someone up." No matter which term is used, they all mean the same thing - making a phone call to someone.