As I don't know the 'tayong dalawa' full story, I can't determine its significance. It could be that 'tayong dalawa' represents two important characters who drive the plot forward. Maybe they are symbols of unity or contrast in the story.
Gabi remains alive in the manga. The plot doesn't lead to her death. It's one of those character arcs where she continues on despite facing various challenges.
Since I don't have detailed information about 'tayong dalawa', I can't tell you the full story. It might be a local or regional story that is not widely known. Maybe it's a story from a small community, a family tradition, or an emerging piece of literature that hasn't gained much exposure yet.
Mount Isa ghost stories likely draw from the area's unique history. The mining history could be a big part of it. For example, the dangerous conditions in the mines may have led to many deaths. These untimely deaths could be the source of ghostly apparitions. People might claim to see the ghosts of miners still working in the shafts, or hear their pickaxes.
Gabi is still alive in the AOT manga. Her fate was determined by the plot and the author's choices, and death wasn't one of them for her in this particular storyline.