Hey welcome to the forums!
Your doc says you have gastroparesis? It seems to me to be one possible cause.
Qs for you:
Does it get worse or better when eating any certain foods? Fatty foods? Carby foods? Raw food? Cooked foods?
Do you have a history of any eating disorders or uncontrolled diabetes?
Where is the lump located, specifically?
Does it improve or worsen after eating or having a bowel movement?