Meanwhile, on Earth

This could be seen as a huge plot-hole in the MCU: If Guardians of the Galaxy Vol. 2 took place before the Civil War, why was there no mention of the strange blue gooey stuff that threatened to swallow the Earth?

But that wasn’t the point, and we can explain it this way: Doctor Strange played with time itself, changing the timeline.

So then were were back on Earth. Leaving aside the catastrophic event the Guardians prevented, we saw that Peter’s grandfather was alive and well, even though the goo almost swallowed him. This small cameo set up the perfect reason for Peter to return to Earth after coming to terms with his mother’s death.