Something that I find interesting (but also sad) is that the first interaction we see between Garp and Ace on page happens when the latter is imprisoned at Impel Down:
The first interaction we have between Garp and Ace's father, Roger, also happens in a prison:
And on this scene Roger also asks Garp to take care of his unborn child.
It gets to a pattern to have Garp first interacting with important people of his life on prison as the same happens between him and his son, Dragon:
Garp not only freed his son from prison but also he was shown to be okay with Dragon quitting the marines. Which I guess probably mean that father and son have a better relationship than the Navy suspects. After all, Dragon entrusted his only child to his father.
Currently, Garp is the one who is imprisoned. I think it would be really fitting to see the reverse of the above scene with Dragon being the one to free his father this time.
I could also see Luffy being the one to free/rescue his grandad. Because Blackbeard who has Garp imprisoned, is Luffy's rival, not Dragon's. Also, Luffy is the protagonist of the story so the relationship between him and his grandfather holds more narrative significance than that of Garp and Dragon.
Perhaps father and son (Dragon and Luffy) could team up to rescue the grandad? Or even accidentally meet while both are there to rescue Garp. That would make a great D. Monkey reunion of three generations.
Lastly, I wanted to share that Garp in captivity looks similar to his grandson, Ace while the latter was also imprisoned: