This should be a simple question:
- Host OS: Arch Linux
- Guest OS: Arch Linux (GNOME)
How can I send Ctrl+Alt+F1 to my Guest Linux OS?
Host + F1, default Host key is Right Ctrl.
Long answer: The quickest path to the answer is actually VirtualBox's excellent manual (yes, some people still write those things these days). You can open the manual with the menu Help -> Contents... and type "ctrl+alt+f" in the search field: this will position you right to the chapter "Typing special characters", with the answer you're looking for. Otherwise, you can also read it online: https://www.virtualbox.org/manual/ch01.html#specialcharacters
Short answer: We tend to rely too much on stackexchange, while we just need to RTFM to find a quality answer. (me included :)
If you have vnc installed, you can run VirtualBox in a vnc session. Then use a vncviewer (e.g., using the F8 context menu in tigervnc's vncviewer) to set the Alt key in a pressed state. Then manually enter Ctrl + Fn to switch to the n'th virtual terminal. "Unpress" the Alt key via your vncviewer.
Not the quickest method, but it works when other methods don't work (e.g., Host + Fn did not work for me in VirtualBox 5.1.26).