The installation of guest additions fails with gnome-terminal version do to deprecated commands in install command.
localhost ~]$ /bin/gnome-terminal --version
GNOME Terminal 3.24.2 Using VTE version 0.48.3 +GNUTLS
localhost ~]$ sudo sh /run/media/djohnson/VBOXADDITIONS_5.1.28_117968/autorun.sh
Here is the command that fails in the shell script:
+ BINARY='/run/media/djohnson/VBOXADDITIONS\_5\.1\.28\_117968/VBoxLinuxAdditions\.run'
+ exec /bin/gnome-terminal --title 'VirtualBox Guest Additions installation' -x /bin/sh /run/media/djohnson/VBOXADDITIONS_5.1.28_117968/runasroot.sh --has-terminal 'VirtualBox Guest Additions installation' '/bin/sh /run/media/djohnson/VBOXADDITIONS\_5\.1\.28\_117968/VBoxLinuxAdditions\.run --xwin' 'Please try running "/run/media/djohnson/VBOXADDITIONS_5.1.28_117968/VBoxLinuxAdditions.run" manually.'
Option “-x” is deprecated and might be removed in a later version of gnome-terminal.
Use “-- ” to terminate the options and put the command line to execute after it.
Option “--title” is deprecated and might be removed in a later version of gnome-terminal.
No protocol specified
Unable to init server: Could not connect: Connection refused
Failed to parse arguments: Cannot open display: 
Most likely, this is due to a lack of dependencies, when I ran into this on CentOS 8 box, I did this to resolve the problem:
dnf install tar bzip2 kernel-devel-$(uname -r) kernel-headers elfutils-libelf-devel make gcc perl
The autorun script should check for these dependencies and report what is missing (and not such a cryptic error message).