C:\Users\veblush\Downloads\ruby>gem install grpc
Fetching grpc-1.30.1-x64-mingw32.gem
Successfully installed grpc-1.30.1-x64-mingw32
Parsing documentation for grpc-1.30.1-x64-mingw32
Installing ri documentation for grpc-1.30.1-x64-mingw32
Done installing documentation for grpc after 2 seconds
1 gem installed

C:\Users\veblush\Downloads\ruby>ruby a.rb
Traceback (most recent call last):
        2: from a.rb:1:in `<main>'
        1: from C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': cannot load such file -- grpc (LoadError)
        8: from a.rb:1:in `<main>'
        7: from C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:156:in `require'
        6: from C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:168:in `rescue in require'
        5: from C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:168:in `require'
        4: from C:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/grpc-1.30.1-x64-mingw32/src/ruby/lib/grpc.rb:19:in `<top (required)>'
        3: from C:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/grpc-1.30.1-x64-mingw32/src/ruby/lib/grpc.rb:19:in `require_relative'
        2: from C:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/grpc-1.30.1-x64-mingw32/src/ruby/lib/grpc/grpc.rb:20:in `<top (required)>'
        1: from C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
C:/Ruby27-x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': Couldn't find or load gRPC's dynamic C core (LoadError)