To use Telegram without a bot, you must work with the Telegram low-level MTProto . There are MTProto libraries for Python , JavaScript, and even for PHP . See examples of getting chat history with these libraries here , here and here.
But it seems that there is no Ruby gem for this - failure, you must either create the MTProto Ruby gem yourself, or transfer this functionality to another service written in the language that the MTProto library has.
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