"create a gmail account in Kasa?"
1. create a gmail account with Google, if you don't have one. Log in using your web browser.
2. enter that gmail address into the Kasa app. You will get an email at that address, with a link to click to prove you have access (own) that email address.
3. Note that the login is not necessarily identical to the login for the TP-Link forum, I do not remember them being the same.

Hi,

I did as you said and got an activation link in Gmail. I clicked on the activation address and it seemed to work fine. But in Kasa it still says "unverified email". When I click on "verify email" I get "account has been activated".

Whats going on?

This is from the thread above yours- maybe it will help.
@Squintyeyed Pug, as it turns out, you may have been right. There is a website: http://www.tplinkcloud.com where you can setup an account and TP-Link support people suggested I try there. Fortunately, it turned out that all I had to do was to log out from my phone App and restart the phone app, logging in with the mail and password I had entered originally. The status was updated to connected and has worked fine ever since.