If you are attempting to send an SMS message via chat in Gmail and are told that the recipient must reply before more messages will be delivered, you will not be able to send your message until they do so. At this time, if they are unable to respond, you will be unable to send additional text messages to this contact.
To prevent abuse, any Gmail user who sends an excessive number of SMS messages from Gmail, without getting any responses in return, will be blocked from sending further SMS messages. Please note, text messaging in Chat is a Gmail Labs feature. Gmail Labs is a testing ground for experimental features that aren't quite ready for primetime. They may change, break or disappear at any time. As such, we are unable to unblock an account once it has been blocked from receiving additional SMS messages.