After hitting submit on my Online Form I want the Form to dissapear and make my Display MediaGallery

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.

Is it possible to Show or Hide a web part using Jquery/Javascript in Kentico ? If that is so, I can write a Jquery function tied to the Submit click event of my form.

you can show/hide web part on the server side using macros/k#. you can show/hide an element of document(DOM) on a client side using jquery/javascript, but I would not do it on the client side on onsubmit event.
I would add my javascript like in the example below: If your form correctly submitted the display text will be rendered (you ll see the alert). instead of alert you can do $('#divMedia').toggle() assuming that your media library is wrapped into <div id='divMedia' style='display:none'>...</div>

@Peter Mogilnitski

Any chance you still have a picture of that example handy? Or could explain what it was.