So, you’ve started using Gmail’s SMS service and exhausted the given free credit limit of 50.
Buying credits is easy. You don’t need to pay Google for it rather you have to pay your mobile operator.
Google started this service in collaboration with mobile operators of different countries. Both share the revenue between them. And process of gaining credits is based on this only.
When you send one SMS from Gmail, one credit is lost. And when you receive a SMS on Gmail from mobile, your credit increases by 5. You’ll gain credits when someone replies to your SMS from mobile phone.
If credit reaches zero then you’ll automatically gain 1 credit within 24 hours. Use this credit to recharge mobile by yourself by repeatedly sending messages from your mobile to your Gmail account. Here you’re basically paying the mobile operator(and Google) to increase your Gmail credit.
When a receiver replies to the SMS sent by you through Gmail, some of his balance is deducted by the mobile operator. In India, Rs 1 is deducted. This INR 1 is the profit that’s shared between mobile operator and Google. For Rs 1, Google provides 5 free credits to the you (who sent the SMS for which receiver relied by paying Rs 1).
So if you want to increase Gmail SMS credit by yourself then keep sending messages from your mobile to your Gmail profile.
The maximum achievable credit is 50, so for gaining 50 credits you need to send 10 messages back paying 1rs each.