Bob is the grandfather of Miguel (by virtue of their time traveling experience).Still no offence but.
That is still incorrect it may well be how you word it I dont know.
But if he and his friend traveled back in time and then try to be their own grandfather's but they mess up and end up as each others' grandfather's, then thats as far as it go's.
He is now his friends grandfather and his friend is now his grandfarther.
However for it to work the way you say it do's and how it is in your diagram they would then need to go further back and mess up yet again by trying to be their own grandfathers, grandfather there by making them their own great great grandfather as you say.
Just saying
Miguel is the grandfather of Bob (same reason).
Since Bob is the grandfather of Miguel and Miguel is the grandfather of Bob, that makes Bob the great great grandfather of Bob.
The same goes for Miguel: Miguel::Bob as Bob::Miguel
I believe you are thinking of time as linear, however in this case - in terms of conception - their time travels created a loop of procreation spanning several generations. All three Bobs and all three Miguels are hypothetically the same in each person's case.