How many grams are there in an ounce?

The gram (often gramme in British English) is a unit of mass who's symbol is g.

Britain has used three different weight systems in the 19th and 20th centuries: troy weight, used for precious metals; avoirdupois weight, used for most other purposes; and apothecaries' weight, which is now defunt and the metric system is used for all scientific purposes.

The use of the troy pound (373.241 721 6 g) was abolished in Britain on January 6, 1879, making the Avoirdupois pound the primary unit of weight. With only the troy ounce (31.103 476 8 g) and its decimal subdivisions retained.

In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.

A possible source of confusion is that gold is often priced on the open market in the more traditional troy ounce
(one troy ounce is exactly 31.1034768 grams, which is larger than the avoirdupois ounce which has a mass of 28.35 grams).

A gold gram is the amount of value represented by exactly one gram of gold.
It is a unit of account frequently used for digital gold currencies.
It is denoted by the symbol "gg", "AUG", or "GAU".

A milligram of gold is sometimes referred to as a mil or mgg.

Therefore, 1 AUG = 1 gg = 1000 mgg = 1000 mil, and 1 mil = 1 mgg = 0.001 gg = 0.001 AUG.
This allows gold holdings and transfers to take place in tiny fractions of a gram (equivalent to a few pence)

1 ounce (avoirdupois) = 28.349523125 grams
1 ounce (troy) = 31.1034768 grams

Which really still doesn't answer the question which is why Cash for Gold pay by the gram for gold.

Beware the people who advertise to buy your gold in ounces, you will always come of worse!