Ok, so you find a freebie or registration form online. They require an email address to complete the deal. Sure, your gmail account may be considered “disposable” but is it really?
Gmail allows the use of “tags” to help sort inbound email. If your email account is firstname.lastname@example.org you can add email@example.com. Many web forms don’t prevent you from using the tags+ when registering.
Additionally, if you are morally deliquent, you can use the same trick to register multiple times, as most web applications treat each new tagged entry as a unique email.