It seems that there is no way to parse tag.
Tag messages are emited on the bus attached to a pipeline but nothing can be done with them.
I see no usefull public method for Gst::Message::TAG. A parse method could be handy.
There was a parse_tag method for the old binding. See rb_gst_message_parse_tag on https://trac.luon.net/ruby-gstreamer0.10/browser/trunk/ruby-gstreamer0.10/src/rbgstmessage.c. Do not know if it can help.