Note that I would no longer advise using any the tools described here. Just use Signal!
That said, I wrote out these instructions at some point so I'll leave them here for now.
Pidgin comes installed with Ubuntu 16.04. To add my existing Google Account, I followed the steps outlined in this Stack Overflow answer.
In Pidgin, add a new account. Set the Protocol to "Google Talk", username to your Google username, and the Domain to "gmail.com".
For the password you'll need to create a dedicated app password. You can do that in your Google accounts Security > App Passwords section. When creating the new app password, set "app" to "other" and call it something like "linux pidgin"-- it doesn't matter what you call it. Optionally, if on a secure computer, tick the "Remember password" checkmark. (Warning: This will mean your new app password will be stored in plain text in
~/.purple/accounts.xml.) Leave "Resource" and "Local Alias" blank.
Once my Google account was successfully added, I installed the Pidgin-otr plugin by running
sudo apt-get install pidgin-otr in terminal. To enable and setup OTR, I followed this EFF guide. That guide also describes how to install the otr plugin through the Ubuntu Software manager if you're more into GUIs (see the early steps of that EFF guide).
To display offline buddies, go to Buddies > Show > Offline Buddies. There are more preferences, like muting sounds, in Tools > Preferences.
Not sure how to disable the pop up notifications yet though.