Grand Master (sometimes abbreviated as GM) is the professional title for a character who has developed a skill to 100.0 points. The character will continue to have the title until reaching 110.0 skill points, at which point they become known as Elder, then 120.0 when they finally become Legendary.
When Ultima Online was first released, and for many years until the advent of the Age of Shadows expansion(which included Power Scrolls), Grand Master was the maximum level a skill could be raised to and was often considered quite an achievement. Power Scrolls later allowed for certain skills to be raised up to a maximum of 120.0 skill points.
An in-game character who reaches GM level gains certain bonuses when using their skills. For example, if a crafter creates an exceptional item when at GM level or higher, they have the option of having their name, or Maker's Mark, inscribed upon it.
The title Grand Master originated in medieval Europe where trade guilds regulated craftsmen. After an aspiring artisan achieved Apprentice, Journeyman and finally Master level in a trade, he could attempt to create an article of his trade that expressed his creative and technical skills at their peak, known as a master piece. This article would then be submitted to the trade guild leaders for consideration. If the article demonstrated peak excellence, the title of Grand Master would be bestowed upon the artisan. This example of a skill tiering was adopted in Ultima Online but also persists to some degree in the modern world in various fields such as plumbing, electrical and cooking.