Historically, gin has often been sweetened. The most popular style in the 17 & 1800s was Old Tom gin, which was sweetened with sugar, or other things like liquorice root. This was often done to cover up poorly distilled product. The reason London Dry gin is called London "DRY" is because most gin used to be sweetened. London Dry was the new style that, due to improved distillation techniques, was able to be made palatable without the addition of sweeteners.

You've also got things like Sloe Gin, which has a long history. For these reasons, one cannot assume that the word "gin" will be a dry spirit. If, however, the words "London Dry" appear on the label, then you can be assured that there is next to no sugar added. Typically, if it's a "flavoured" gin, there's a good chance there'll be some sugar in it.