You can probably create a custom macro method where you can return
System.Web.UI.Page.IsPostBack property and set visibility for media gallery web part based using this custom macro method, but you have to keep in mind that a postback may occur, but for example some fields validation can fail.
As far as I understand you want to show media gallery only if form submission was successful. Probably 2 page solution will be better here. 1st page with a biz form and 2nd page with a media gallery and in the biz form settings you can specify "After the form is submitted:" redirect to page (2) with gallery.
Both pages can use the same master.