Gold is one of the most malleable and ductile metals available and is ideal to be used in jewelry making because it makes for easy sizing. Before many of the modern metals like titanium and palladium came to prominence in recent years gold was the to go to metal for wedding bands of all types. Jewelers have been familiar with manipulating this metal since time immemorial and if you bought a ring or wedding band made from a metal like palladium you may have been hard-pressed to find a jeweler willing to work with that metal whether for repair or sizing.
The basic technique used for sizing gold rings is cutting and soldering using a flux solution to prevent the oxidation of alloys and to speed up the process of soldering gold together using a solder material. Unlike the platinum group of metals (which includes platinum and palladium) which are 95% pure and can actually be welded together without the use of solder, gold alloys which are only usually 58% to 75% pure require the use of solder to form a bond.
The working temperature of gold is also much lower than platinum and palladium and melting temperature is usually around 1500°F whereas the other two metals require in excess of 2500°F to be welded. This makes working with gold for repair purposes and setting purposes as well as sizing a much less expensive metal for jewelers to work with in terms of time required and equipment required. Platinum and palladium are also much harder metals and require many more types of grit to be used for polishing.
Gold alloys are also easier to stretch in case the ring needs to only go up a quarter size to a half size. Platinum although a malleable metal is much tougher to size up this way because it needs to be hammered more and prodded more for it to stretch. It is a much denser and heavier metal as well as harder. Other modern metals like titanium and tungsten are even more difficult to work with and most jewelers are not willing to touch any piece of jewelry made from them.
Despite its expense because of the current high world gold pricing it is still the easiest precious jewelry metal for a jeweler to work with because its use in jewelry has been so ubiquitous. Gold does tend to conduct heat readily so sizing and repairing wedding rings that have sensitive gemstones has to be done with caution and preliminary set up. However it is still easier for a jeweler to do basic repairs to rings made from this precious metal more so than any other.