Gown wearing is highly unusual in the U.S. Outside of bridesmaids (the women who stand with the bride), girls at special high school events (like homecoming and prom) and formal balls, women in the U.S. don’t wear gowns. I haven’t worn one in over 20 years!
For most American weddings, a woman can wear a nice skirt, slacks, dress, or suit. The color typically doesn’t matter, although many avoid wearing white since that’s normally what the bride wears. As an international, it would also be appropriate to wear dressy clothes from your home country.
Some Americans completely dispense with formal wedding attire (and weddings). These days, people hold weddings at the beach, in hot air balloons, and anywhere else that they think might be fun. In those cases, the wedding invitation should indicate what to wear.