Gap between implant crown and gum - do I need to refix it without gap? (Photo)

It is in the posterior region and not visible, so if you are ok with the appearance [silver color], then no need to worry. But if you are concerned about looks, choose esthetic [tooth colour] abutment.

Hi:There is no danger in your situation, however you have to decide if this is aesthetically acceptable to you. The only reason to fix is is for aesthetics!Best,Dr. Maddahi

The abutment is seated on an implant that is too coronal or too tall. Assuming the abutment and implant fit together nicely, you still need to have him redo this implant. It is completely unacceptable. If it was placed years ago and your bone and gum has naturally recessed then it would be a different story but you mentioned it was recently done. Not only is it very ugly but it looks like a life long food trap that is not only annoying but can cause bone loss around the adjacent natural teeth.  

All depends on the type of implant placed.  There may be a reason why the crown is above the gumline, such as keeping the area extra clean and safe from getting any food particles/plaque from potentially hurting the implant underneath.  
An x-ray would help seeing how the crown fits on top of the implant.