These instructions are problematic.
1. chattr doesn't work on symlinks (but not needed anyway - +i is not set)
2. dhcp does not overwrite resolv.conf
3. not sure if you want to advise manually editing resolv.conf
This command doesn't rely on dns:
UWT_DEV_PASSTHROUGH=1 curl --tlsv1.2 --proto =https -H 'Host: check.torproject.org' -k https://126.96.36.199
If it doesn't work, then you have no TCP connectivity in your Gateway VM.
You need to install a non-Whonix VM - linux, windows, etc - and see if networking works in virtualbox at all. No point discussing further until you do this.