What is the ruling on showering from 18:00 GMT until the Maghrib prayer?

It is permissible to perform ghusl at any time of the night or day, because the default ruling is permissibility, and there is no evidence to prohibit ghusl after Asr, or before Maghrib, or at any other time. What has been reported from some jurists regarding the dislike of entering the bathhouse just before sunset or between Maghrib and Isha has no evidence to support it. The correct view is that it is not disliked; rather, it is permissible.