Please have a look into the manual at
http://download.virtualbox.org/virtualbox/5.0.20/UserManual.pdf. In Section "4.2.2 Guest Additions for Linux" is described that Guest Additions are supported for Oracle Linux, Fedora, Red Hat Enterprise Linux, SUSE and openSUSE, as well as Ubuntu. On the following pages is also described how to install it on Mandrake Linux, CentOS, and Debian. I am really not an expert in Linux, but I think that all these Linux distributions cannot be summarized as GNU/Linux. Of course, Remix OS is not a supported Linux distribution, but it is an Android distribution, and I believe that Android is based on a kind of Linux distribution (though I am not an Android expert, too). Therefore I have the hope, that there exists a way to make Guest additions run under Remix OS.