Greg most likely learned how to read tarot cards from his mother, who at one point was a professional psychic.
On his website Evolutionary Saga, Greg posted a 2005 email about his tarot reading he did for his friends and family. Unfortunately, the archived version of the email is cut off so his findings are no longer known. (Source)
In a September 2011 Speaks video, "I'M AMAZING...ly messed up", a viewer was surprised to see a tarot deck on Greg's bed. (Seen top right.) ($ource) This caused a bit of a stir because most viewers assumed Greg was against something like tarot reading because of his strong Atheist stance. Some speculated it belonged to his at the time ex-girlfriend Shiloh because it seemed so odd for him to own the deck, hence the text on the image.
One person that claimed to have been on a Skype basis with Greg in 2011 claimed that he would do tarot readings for them. (Source)