Here's a little blurb from the informational plaque at the Gallery:
"Between Schillingbrücke bridge and Osthafen, the political boundary to West Berlin ran south-westwards along the bank of the River Spree on the Kreuzberg side. The entire water surface belonged to East Berlin. The East German leadership had the barrier constructed above ground along Muhlenstraße. Here, you can see a part of the interior wall- now known as the East Side Gallery. In 1990, 118 artists from 21 countries painted the Wall here. This act, which would have only been allowed in West Berlin until then, gave expression to the joy widely felt at the falling of the Berlin Wall. The open-air gallery, which is 1.3 km long, is the longest stretch of the rear barrier to be placed under a preservation order in Berlin."