To fix it, you need to edit admin/GForum/Config/Data.pm and search for 'post_attachment' (with the 'single quotes'). It has a value of '4', but the value should be '3'. 4 means the user has to have reply access - 3 means the user needs post view access.
Alternatively, you can wait until tomorrow or Tuesday when we will release 1.1.1, which will fix your installation. Numerous other bugs are also fixed.