Is Piper’s Insta our sole source for Gillian herself being in Costa Rica or are there other pieces of evidence? Because piper can certainly travel solo so we would need more ??

Oh no, there’s no doubt she was in Cost Rica. There was pieces of evidence outside of Piper’s IG, and most of all, there was a sighting from someone working for a touristic tour that Gillian did with her family.
Seems like she left, though.