Open animation window. For the first frame unhide the smallest swing layer as it will appear the farthest.



Duplicate frame. Unhide the next swing layer that should be a little bigger than the previous one.



Keep duplicating frames and unhiding the sequential duplicates from farthest to nearest.



Create more frames and unhiding the sequential duplicates, this time from nearest to farthest creating a reverse sequence.



Press Ctrl+Alt+Shift+S to save an optimized animated GIF.