It is written using GTK and comes with a fully featured, easy to use GUI. So, if you want to use GyachI yahoo messenger client in Linux... then the app itself is well equipped without a doubt. From things like changing fonts, colors, account settings, better security, web-cam support, address-book, etc... it does come with a lot of features of its own.
So, if you want to install it in Ubuntu 11.04, then open your Terminal and issue the below command.
sudo add-apt-repository ppa:adilson/experimental
sudo apt-get update
sudo apt-get install gyachi
But remember, as said before, this is still at experimental level.. so use it at your own risk.