This package's original identifier was com.atteneder.gltfast (see Unity Fork for details). You can still use/install it with that identifier.
Via OpenUPM
The recommended way to install is to download and open the Installer Package
It runs a script that installs glTFast via the OpenUPM scoped registry.
Afterwards glTFast and further, optional packages are listed in the Package Manager (under My Registries) and can be installed and updated from there.
If you want to clone the GIT repository and installed the package locally (which is useful if you intend to develop glTFast itself), you have to checkout the branch openupm.
The package identifier in the main branch was changed, which likely leads to errors if you've cloned it before and pulled that change. In that case either switch to the openupm branch or transition to the new package identifier.