To install GTPack you would need to choose “Application” as “Type of Item to Install”.
- Download and unzip GTPack
- Open Mathematica. Go to File -> Install…
- Type of Item to Install: “Application”
- Source: “From Directory”; Pick the downloaded GTPack Directory in the file explorer
(An alternative manual way of installing GTPack is explained here: https://gtpack.org/tutorials/install-gtpack/)