That looks like the piece on the left was compressed or crushed slightly on the edge. I wonder if putting a towel on it and pouring some boiling water on would get it to expand back some, or maybe even all of the way. Then you could probably just slip a little TB2 or GG in there to do the rest.
If that doesn't work, I think the GS is probably the best way.
You could try to cut a wedge shaped sliver of foam to glue into the gap, but if it were me I would go with the GS. Squirt a small amount in there and if it doesn't go where you want it push it around with one of those fake membership cards that come in the junk mail all of the time (with disposable rubber gloves on) and put it where you want. You don't really want to spread it or knock it down (like you would icing on a cake), more like putting meringue on a pudding pie (don't deflate the bubbles).
You could also get some epoxy resin and filler (micro balloons, cab-o-sil, wood flour, etc.), but that would be kind of expensive compared to stuff that you already have.
I would not try to fill that with spackle. It looks to be too big of a gap and it doesn't have any strength to speak of.
Good luck.
