Gold Membership or (GM) is a Promotion which gives you bonuses and extras
+50% Exp and Money
+50% UC/R/VR/ER/L Rate
+20% Shiny Rates
GM Players will have a Yellow Name in the chat
You can buy 7 Day Gold Membership or 30 Day Gold Membership from the Credit Store. Another option is to buy it from the other players either in the trade chat or global market. Some achievements can give gold membership as prizes as well as Tournaments. You can also get GMs from Easter Eggs, Mystery Boxs or Christmas Presents. They also can be dropped by a shiny pokemon.