In order to unlock a Samsung T101G you need an unlock code. It sounds simple enough, but you're unlikely to find a way to do this for free on the internet.
There are many websites on the internet that claim to provide you with a Samsung T101G unlock code, but the majority (if not all) of these websites require you to pay for them.
This might work, but it's just as likely that you'd end up buying a code that doesn't work and then you're back to square one.
So what should I do?
I'd recommend that you get in touch with Samsung. Their customer service team has recommended customers to do this in the past in order to unlock their phones. This option may also come at a cost, but if it does, you know it's guaranteed to work.