I don't have my sources here at work, but here goes:

Glaidus is simply Latin for sword. The gladius used by the Roman army did not remain the same over time. Initially adopted around the time of the Punic wars (but possibly much earlier), the original Gladius Hispaniensis was longer and heavier than the later versions, with a pronounced leaf shape. This was replaced sometime in the 1st Century BC by the Mainz style gladius, shorter but still keeping the leaf shape. The Pompeii style, with its straight sides and triangular point, starts showing up in the 1st century AD, along with the Fulham style, which resembles a cross of the Mainz and Pompeii styles. The Pompeii style dominates later archaeological finds. Some other style swords are found at later sites, but it's not clear if they're simply swords that lasted that long or newer production.

Sometime in the 3rd Century, the Roman legions began transitioning to the Spatha, a longer sword of Celtic origin initially used by the auxilla and cavalry. This probably reflected both a change in the demographics of the army as well as they type of enemy they were fighting.

Obviously the basic size and purpose of the gladius didn't change much from the adoption of the Gladius Hispaniensis until the Spatha came into regular use in the infantry. The reason for that is that the basic formation and tactics of the Roman Army didn't change much after Marius' reforms. A short stabbing sword was ideally suited to their style of fighting close together, where a longer weapon requires more room to use.