Timber merchants refer to spruce and pine as whitewood and redwood
respectively. For joinery, you want redwood. Whitewood is more
commonly used for constructional work that will not be seen.

T&G boards are perfectly acceptable for a gate - usually built up as a
"ledged and braced" door (with the classic Z pattern of braces on the
back - often seen on sheds) or used to board over a "framed" door
(where more substantial timbers are morticed and tenoned together to
form the frame).

A prettier appearance is T,G & V - where the V is a v-groove formed by
a bevel on each edge of the board.

Either way, think about how you will ensure rainwater drains away from
the door/gate well, and doesn't sit on the ledges or work its way into
the joints.