Suppose There is a Gmail account : email@example.com
with password : abcd
You can log on into gmail using any combination of a dot.
and so on
For all the above cases the password will be same abcd.
That means gmail does not count periods (.)
So you can use this trick for using the same email for various GPT, PTC, Twitter accounts as they interpret all the above emails as different but gmail considers it the same.