Default installation of D8 treatment of .gif accesses?
By default drupal serves a different 404 page to save computational effort if it  is matching the following regex:

'/\.(?:txt|png|gif|jpe?g|css|js|ico|swf|flv|cgi|bat|pl|dll|exe|asp)$/i'


You can configure this in the settings.php (Fast 404 pages) or in the config('system.performance')->get('fast_404.paths');