Antonyms for the phrase "gathers momentum" can range from words that describe a lack of progress or action, to words that suggest a decline in energy or activity. Some possible antonyms include phrases such as "loses steam," "comes to a halt," "fades away," "loses traction," or "hits a roadblock." Other options might include words like "retreats," "recedes," "shrinks," or "diminishes," all of which suggest a decrease in intensity or vitality. Overall, the antonyms for "gathers momentum" demonstrate a wide range of possible directions or outcomes, all of which can have a significant impact on the course of events or actions.