Wasn't it called Google Talk? And it was based on Jabber/XMPP, which let you use any compatible client.

I hate that they dropped support for it, choosing to go with a proprietary protocol without any public APIs.

It was Talk, but I think before it was a separate offering/app, it was just "chat on Gmail", and hence, "GChat" took off as an informal name for it.

> which let you use any compatible client

Which means that third-parties can easily capture large parts of the "friends" graph with just a small number of users consenting to give them data.

You're also now entrusting your communications to whatever clients other people are using without any chance/need for you to consent to their privacy policies.

Yep, sorry, Google Talk. Edited my message

Also it was reliable and interoperated fairly well despite other chat providers' sporadic attempts to build moats.

For me Gtalk was the Yahoo Messenger of late 2000s. Me and my friends almost exclusively used Gtalk since it was very lightweight, it worked well, and you could use any of the large number of free clients with it.

Google had Slack too, two years before Slack, in the form of Wave.