what method should I use for removing paint from GFX and bumpers for repainting? Sanding, Chemical or Blasting?

Sanding would be fine. I used 180 to remove the paint from my bumpers.

did u hand sand it off or did you use an electric or air sander?

i generally hand sand due to all the curved surfaces, 150 then 220 or 300. if you don't care about removing the last bit of paint, 220 should be ok.

I prefer hand sanding over the power sander.