First Easter egg takes place in rdr1, in the first mission John meets His Fort Mercer guide "Jake". Jake asks "You must be John Marston" and John replies "sometimes", meaning his fake name "Jim Milton" in Rdr2 is a Easter egg.
2nd Easter egg takes place in The Epilogue of Rdr2, If you rent a room in the Blackwater saloon inside the room you will see a blanket with looks identical to John's Mexican Poncho awarded after finishing the Mexican Chapter. And finally the encounter, I found Gavin's friend in McFarlands Ranch, Hennigans Stead, New Austin. , If you choose to great Gavin's friend. John asks Gavins friend if he is still looking for Gavin. Gavin's friend replies stating how he's been looking for "7 , 8 Years now!". John replieing " I would've stopped looking after 7 or 8 days" have you guys experienced this