I have GSTIN, and if I purchase Book Keeper, Will I be able to claim GST

Yes, definitely,

If you have provided GSTIN at the time of registration, you will get B2B invoice else you will get B2C invoice.

If you forgot to provide GSTIN, don't worry, you can revert (within 24 hours) on invoice (which you have received) with your details, we'll share revised invoice.