Is the Dawn Prayer to be repeated after feeling something (madhy, manyy, or water) exit the penis onto clothing following a ritual bath (ghusl al-janabah), knowing that the ablution (wudu') for prayer was performed after a minor ritual impurity (urination)?

If the sensation of something exiting is merely a doubt, then it has no ruling.

However, if you are certain that something has exited, then re-performing ghusl (definition) is not required. Wudu suffices, along with purifying whatever the exiting substance has touched.

If you performed wudu after the exit of the substance, but forgot to purify your body and clothes from it, then your prayer is valid.

But if you did not perform wudu after the exit of the substance, then you must repeat the prayer. Forgetting wudu is not an excuse, because prayer is not valid from one who is in a state of ritual impurity (hadath (definition)).