i finished my first part today using GC50 gel coat. Poblem is that the part got hazy and on an yellowish side.

Can you perhaps measure your gelcoat thickness? I'm not sure how sensitive the GC50 is to thickness (Warren?), but I know some gelcoats need to be pretty spot on 25 micron. Perhaps your issue is not thick enough rather than too thick, leading to undercuring of the gelcoat? I sort of doubt that though.