GeoTrellis is used as a library dependency, and can be included via your Scala build process.
To use GeoTrellis, add the following to your build.sbt:
libraryDependencies += "org.locationtech.geotrellis" %% "geotrellis-raster" % "1.1.0"
resolvers += "LocationTech GeoTrellis Snapshots" at "https://repo.locationtech.org/content/repositories/geotrellis-snapshots"
There are many subprojects for GeoTrellis. See the README at https://github.com/locationtech/geotrellis for more information.