Before downloading GoCertius I want to know if I can certify existing Telegram groups. Is this possible?

It is not possible to certify Telegram chats that were not created using GoCertius.

We firmly believe in rigorously certifying all messages. If Certy has not been recording all messages and actions since the creation of the chat, there is a possibility that the chat may have undergone changes that we would not be able to certify. We would also be unable to rigorously certify the point in time when the existing messages were created.

In addition, on a technical level, to be able to correctly receive all messages, attachments and actions of a group, the group must be created in very specific way by GoCertius using the Telegram API.