http://grammatica.percederberg.net
If you dig further you will see the grammar file is for Grammatica, which only supports Java and C#.
Either you extend Grammatica to support Ruby or you will have to switch to other approaches. Note that Mibble is released GPL and a commercial license. If you are going to reuse the grammar file, make sure you meet the licensing requirements.
Our open source project, http://sharpsnmplib.codeplex.com uses ANTLR 3 to parse MIB documents. The grammar file is released under BSD 3 Clause. As ANTLR does support Ruby, http://www.antlr.org/wiki/display/ANTLR3/Antlr3RubyTarget, you might reuse our MIB grammar.