How to 'gradient out' to white?

[Ofnuts]

• Layer >Transparency>Remove alpha channel (if greyed out, it's already done)
  
• Use the Measure tool to evaluate the minimum distance between the shoes and the current white background (around 30px in the image you posted)
  
• Use the Wand tool to select the white
  
• Select>Grow by half the margin measured above (15px)
  
• Select>Feather by the margin measured above (30px)
  
• Edit>Clear or [delete]