Is accessibility testing different from GUI testing ?

Yes,

Accessibility testing is different from GUI testing.

GUI testing: Test for Graphical user interface. How site looks? Test for site design in all browsers, design should not corrupts.

Accessibility testing: Test for how site is usable for partial or complete disabled persons. For example: if a user has eye problem, weak sight. Then is there any hearing feature for  words displayed on site.

I think now difference is crystal clear. Any question, please comment.