Congratulations on your allotment at IIT KGP. If you have freezed the seat and got the seat allotment letter after seat acceptance fee payment on the Josaa Portal, wait for the Document Verification Date. I believe this year the process is Online and you will be given a particular slot.
Post Document Verification and all the JOSAA rounds you will receive an email from IIT KGP/ERP IIT KGP for formal institute registration and identity verification on the official SAP/ERP Portal and you have to act accordingly. The Email will also give you instructions regarding Institute fee payment and Hall Allotment. You will finally get your Institute Roll Number in the ERP Profile. Till then Chill and Relax, perhaps check the IIT KGP ERP Portal as I believe a notification PDF might be released regarding New UG Admissions and Rules/Regulations.
I believe your classes and induction process might start online, however as planned in the Institute Calendar of IIT KGP, the New admits and their parents might be allowed to visit the campus and complete few formalities in December, subject to Covid Conditions and Transportation facilities.
All the best.
For more information about IIT KGP do follow the link below