Check it out in Paclet Repository.
Install from the paclet repository:
PacletInstall["Wolfram/GeometricAlgebra"]
Or locally:
PacletDirectoryLoad["path_to_paclet_directory"]
<< Wolfram`GeometricAlgebra`
e = GeometricAlgebra[3];
r = 2 e[1] + 3 e[2] + 4 e[3]
r ** e[1, 2]