You can certainly make Vehicles as
characters using GCA. You can add appropriate traits to a character to give it vehicle-like abilities, from Wings to No Legs (Wheels) and the Machine meta-trait. However, these are characters, not vehicles. There are no rules for designing actual Vehicles in GURPS 4th Edition; even if there were GCA remains first and foremost a
Character designing utility.
All that said, if you are interested in Vehicles you may want to check out the GURPS Spaceships line. Combined with the rules in Pyramid 3-34 Alternate GURPS you can make most larger vehicles with reasonable degrees of consistency. Using my
GURPS Spaceships Design Spreadsheet you can easily make most any design using those rules.