What I've usually done is install the T&G directly to the frame, before installing the panels. It's much easier to nail it to the frame from the back side, as you don't have to blind nail it through the tongue of each board. I also highly recommend either buying pre-finished T&G, or applying the varnish to the paneling yourself before you install it.
With the drywall, I've done it two different ways:
One is similar to what I described for the T&G: attach it directly to the frame (using screws through the back side of the drywall) before you install the panels. This means that there are only a few screws holding it up until you get the panels on. Once the panels are up, screw the drywall securely to the panels from the inside.
The other method is to install spacers to the outside of the frame, and then install the panels. The spacers create a gap between the frame and the panels, so that you can slip the drywall in between. Although it takes a little longer, it's less likely to rain if you do it this way
.
I have never tried attaching the drywall to the panels before installing them. I think it would be very easy to damage the drywall while the panels are being installed.
Good Luck with your project! CB.