It's said that Gothic had no umlaut, but there would seem to be, although I'm sure it's not, signs of a-umlaut. The digraph au in Gothic is thought to have represented three different sounds, one of which was a short o, as in Gothic waurd or Dauhtar. I've read that Proto-Germanic did not have a short o, and that in words like gold and horn, a short u was lowered by a following a. Would the short o in Gothic be the result of something else, and if so, what?
It's the result of lowering of /u/ before the consonants /h~x/, /hʷ~xʷ~ʍ/ and /r/. According to Winfred P. Lehmann in Gothic and the Reconstruction of Proto-Germanic, short /o/ and /u/ (and short /e/ and /i/) were nearly in complementary distribution in Gothic, but further sound changes that occured after the process of lowering created some exceptions to this general pattern, such as aíþþau "or" and hiri "here".