Only Qubes 4.1 (the upcoming version) will have this fixed. But that release is still far into the future. So for now, the solution is the following:
Opening a Terminal in DisposableVM (permanent solution)
A user suggested a solution that solves the issue permanently (i.e. one can directly start gnome-terminal – the terminal application).
But that may be too advanced for some. See bellow for an easier workaround:
Opening a Terminal in DisposableVM (workaround)
Open the xterm application (instead of the gnome-terminal). And you can use that. However, it is a bit more limited as a terminal. So you can open the gnome-terminal from there by typing gnome-terminal Enter from there.
Opening File Manager in DisposableVM
open xterm application and type nautilus Enter.