The one that started it all. You play as Gubble D. Gleep, a purple alien who finds out another race has taken over his planet. You fight back by progressing through a series of levels where you must clear boards of screws, nails, and other objects by using tool bits found in each level. You pick up the tool bits and hover around the board removing parts while avoiding enemies. Each level is referred to as a "zymbot." Occasionally there are bonus levels you can play through for additional points.

The MobyGames page for Gubble 1 lists a 1996 release in parts, and while I have had the US date updated to 1997, it doesn't sound like it was released anywhere in 1996. From emailing the creator of the game through the Gubble website, the game was completed and ready by the end of 1996 but was delayed for a few months due to the slowness of Randomsoft.

I believe that the PS1 version is still available for sale digitally on one of the Playstation stores. It's on Steam as well in the updated release that builds on the Gubble HD release for iOS with additional fixes. This is the version I would suggest anyway if you were looking to check the series out. The quality of life updates that let you save anytime are nice and all the money goes to the studio that created the original game.