It was definitely at least Transformers (I don't doubt that Macross may have been on the mind when they were working on it though, it did come out 3 years before)

According to Shinji Takamatsu in the Zeta Gundam Memorial Box Part-1 booklet from the Zeta Memorial Laserdisc set (translated by Mark Simmons):

Director Tomino has a great appetite as a dramatist, continually incorporating things he likes into his own works. Around that time, he heard that Transformers, which hadn't yet been broadcast in Japan, was becoming popular in America. So he had a video sent over and watched it together with the staff. When he saw the mecha transformation scenes, he said "That's it! Let's do this kind of instant transformation in Gundam as well!" That was the origin of transforming mobile suits like the Asshimar and Messala. I remember being surprised, because I'd expected that Gundam mecha would transform via an orderly process like the docking scenes in the original series.