Comment by TimBurman

You could try logging into Gmail just to the point before the SMS is sent, then phone the number you used to have and speak to the current owner. Apologize that you forgot to take the number off your account and so they are receiving text messages meant for you, but you would like to get one more message from them and they won't be bothered anymore. Then you will switch to a new number on your Gmail and they will not receive your messages. Many people will be happy to help a stranger if you are nice and polite to them.

The other option is to contact the phone company and explain, asking an open ended question if there is any way they could help you, with the permission of the current owner of the number, to get one more text message and move your Gmail to a new number. It doesn't sound in any way like you are trying to pull a scam, so they might help.