This is one of the three major roles of the statue, as revealed in mid April when more details about Project "Real G" were published. The other two roles are Gundam 30th Anniversary celebration and to support Green Tokyo Project.
This is however, the first time the sticker was revealed.
Another reason to visit the park again for some? ^^
And another reason for Bandai to release a new version of HG 1/144 Gundam Ver. G30th as well? Let's hope not. ^^
Images from Green Tokyo Gundam Project.