G scale trains were created large so they could be run in large spaces like a garden. Since the rails are woven in and around landscaping, it's easier to leave them outdoors year round. Yes, the trains can be run any time of year, but the tracks are subjected to all kinds of weather conditions. So the tracks are built rugged and sturdy to stand up to worst that the weather has to offer.
Bachmann G scale trains are modeled after North American narrow-gauge railroads. The rail spans and rail height on the track were scaled proportional to real life narrow-gauge railroads. Other G gauge trains were modeled after standard scale trains (Burlington Northern or Santa Fe).
Paul D. Race manages "Family Garden Trains" a website devoted to reviewing G scale garden trains and tracks. Since 1996 he has become the "go to" person for questions about G scale railroading and garden track issues. Race recommends that beginners "consider installing a small test loop outside. This is a temporary 1-2-year installation to give you the general feel for what it is like to have a garden railroad." He also suggests setting it up in a location separate from the permanent layout sight so train operation can continue during permanent layout construction.
G gauge track has a rail span of 1.775 inches or 45mm and is called code 332 because the rail height is .332 inches. The track scale (which is different than track gauge) is 1:20.3 up to 1:32, meaning 1 foot of 1:20.3 G scale track is equivalent to 20.3 feet of real life track. Most G scale trains can handle a 2' radius curve, but as a general rule Garden Railways Magazine suggests using a 7' to 8' radius because it creates a more realistic look when trains navigate the curves.
Most G scale track is made from brass because it resists weathering. However, rising copper prices mean more expensive brass tracks. One way to lower costs is to use code 250 (1/4" rails) track instead of code 332 track. It looks more like the prototype but isn't as durable. Another cost effective alternative is aluminum tracks. Weathering creates aluminum oxide which conducts electricity (brass oxide doesn't). Aluminum rails come in flex track, produced in curveable track sections which makes it easy to create free-form curves, though they need a heartier roadbed, such as concrete.
Laying track outdoors will take a little work. First, excavate 2-3" deep channels where tracks will be placed. Next, pack the channels with crushed rock up to the bottom of the rail ties. Make use of rough-edged rocks that will lock against each other when tamped into place with a stick or brick. Next, position and level the track over the ballast bed. Then place a bit more ballast between the rail ties and tamp it into place to hold the rails firmly in place. Ballast may need occasional re-tamping as it will shift due to seasonal changes.