I make a number of doors each year for exterior use with a TGV covering.

The issue with the off the shelf gates and doors that are available will be that they do not have a plywood back and the TGV boards will open up and split with exposure to the elements and in  particular sun.

Another weak point with cheap and not so cheap off the shelf gates and doors is that they are mass produced and literally stapled together with no or very little glue and certainly no Oak plugs to prevent any movement of the mortise and tenon joints.

Usually the above will give you a service life of perhaps 5 years even with the most up to date paint or stain systems that are available.

So it’s a case of buying cheap buy twice or having one made in the first place that will last.

You can see in the accompanying images that certain future proofing strategies used in the manufacture will pay off over time .

The ply back onto which the TGV boards are glued to and fastened. This will prevent movement,splitting and bowing.

The correct depth of the tenon without it being a through tenon thus eliminating end grain exposure. End grain is a weak point when it comes to rot. 

The fitting of Oak dowels to prevent opening of the joints

Manufactured using joinery grade Redwood  using  3rd or 4th grade. The link below will explain the various grades of Redwood.

Buying softwood and softwood grades | Carpentry Services Southampton (southamptoncarpentry.com)

A combination of the above will result in a TGV faced gate or doors that will give you many years of service.