Pidgin

Installation

sudo apt install pidgin pidgin-otr
pidgin&

Configuration

General configuration

Tools > Preferences > Show system tray > Always
Tools > Plugins > [x] OTR

Account configuration

Google Chat

Use this plugin, which you have to build yourself (which is easy):

sudo apt-get install -y libpurple-dev libjson-glib-dev libglib2.0-dev libprotobuf-c-dev protobuf-c-compiler git make;
cd ~/apps
git clone https://github.com/EionRobb/purple-hangouts && cd purple-hangouts;
make && sudo make install

Open pidgin and add an account. You will now have Hangouts as an available login protocol. Once you connect to the account, you will get a prompt requesting your oauth_code. It will take you to a video explaining how to get it. The steps are:

Go to this account link with devtools open.
Grab the cookie value from devtools > Application > Cookies > https://accounts.google.com > oauth_code, and paste into Pidgin.

You can now do P2P OTR communication.

NOTE THAT GROUP CHAT IS NOT (YET) SUPPORTED. I can't even get the account to show me the Google Chat rooms, unencrypted. That means THIS IS BASICALLY A FAIL.