README を読むと apt-get で ruby1.9.1 ruby-gnome2 libnotify-bin を入れてねって書いてあるけどうちの Ruby は rbenv で入れてあるものだし…　という状況を解決．

sudo aptitude install libgtk2.0-dev
gem install gtk2 --no-ri --no-rdoc

これだけ．後は ruby mikutter.rb で動く．

補足

うちの環境は Rails のために build-essential zlib1g-dev libssl-dev libreadline6-dev のパッケージも以前に入れていたので，ひょっとしたらこれらのうちどれかも必要だったのかもしれない．

参考

Ubuntu に mikutter をインストールする | rb.blog.pasberth.com

エラー

何もせずに ruby mikutter.rb したとき

/home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- gtk2 (LoadError)
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/local/src/mikutter/core/mui/gtk_skin.rb:3:in `<top (required)>'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/local/src/mikutter/core/miquire.rb:79:in `miquire_original_require'
        from /home/username/local/src/mikutter/core/miquire.rb:76:in `file_or_directory_require'
        from /home/username/local/src/mikutter/core/miquire.rb:57:in `block in miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `each'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:13:in `miquire'
        from /home/username/local/src/mikutter/core/user.rb:4:in `<top (required)>'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/local/src/mikutter/core/miquire.rb:79:in `miquire_original_require'
        from /home/username/local/src/mikutter/core/miquire.rb:76:in `file_or_directory_require'
        from /home/username/local/src/mikutter/core/miquire.rb:57:in `block in miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `each'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:13:in `miquire'
        from /home/username/local/src/mikutter/core/service.rb:5:in `<top (required)>'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /home/username/local/src/mikutter/core/miquire.rb:79:in `miquire_original_require'
        from /home/username/local/src/mikutter/core/miquire.rb:76:in `file_or_directory_require'
        from /home/username/local/src/mikutter/core/miquire.rb:57:in `block in miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `each'
        from /home/username/local/src/mikutter/core/miquire.rb:56:in `miquire'
        from /home/username/local/src/mikutter/core/miquire.rb:13:in `miquire'
        from mikutter.rb:28:in `<main>'

libgtk2.0-dev を入れずに gem install gtk2 したとき

Fetching: pkg-config-1.1.3.gem (100%)
Fetching: glib2-1.1.3.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing gtk2:
        ERROR: Failed to build gem native extension.

        /home/username/.rbenv/versions/1.9.3-p194/bin/ruby extconf.rb
checking for -Wall option to compiler... yes
checking for -Waggregate-return option to compiler... yes
checking for -Wcast-align option to compiler... yes
checking for -Wextra option to compiler... yes
checking for -Wformat=2 option to compiler... yes
checking for -Winit-self option to compiler... yes
checking for -Wlarger-than-65500 option to compiler... yes
checking for -Wmissing-declarations option to compiler... yes
checking for -Wmissing-format-attribute option to compiler... yes
checking for -Wmissing-include-dirs option to compiler... yes
checking for -Wmissing-noreturn option to compiler... yes
checking for -Wmissing-prototypes option to compiler... yes
checking for -Wnested-externs option to compiler... yes
checking for -Wold-style-definition option to compiler... yes
checking for -Wpacked option to compiler... yes
checking for -Wp,-D_FORTIFY_SOURCE=2 option to compiler... yes
checking for -Wpointer-arith option to compiler... yes
checking for -Wswitch-default option to compiler... yes
checking for -Wswitch-enum option to compiler... yes
checking for -Wundef option to compiler... yes
checking for -Wunsafe-loop-optimizations option to compiler... yes
checking for -Wwrite-strings option to compiler... yes
checking for rb_define_alloc_func() in ruby.h... yes
checking for rb_block_proc() in ruby.h... yes
checking for new allocation framework... yes
checking for attribute assignment... no
checking for Win32 OS... no
checking for gobject-2.0... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/home/username/.rbenv/versions/1.9.3-p194/bin/ruby
        --with-pkg-config
        --without-pkg-config
        --with-override-variables
        --without-override-variables


Gem files will remain installed in /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/glib2-1.1.3 for inspection.
Results logged to /home/username/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/glib2-1.1.3/ext/glib2/gem_make.out