gentoo-overlay/sys-apps/gnome-disk-utility/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<herd>gnome</herd>
<use>
	<flag name='fat'>Include FAT16/FAT32 support
		(<pkg>sys-fs/dosfstools</pkg>)</flag>
</use>
</pkgmetadata>