well, that depends on the project in question. Some projects have a *.gemspec file in their root directory. In that case, it would be
gem build GEMNAME.gemspec
gem install gemname-version.gem
Other projects have a rake task, called "gem" or "build" or something like that, in this case you have to invoke "rake ", but that depends on the project.
In both cases you have to download the source.
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