8 weeks post-op and I can see that there is a gap/space in my gum where the implant is visible, am I rejecting the implant?

Hi, when the chin implant is placed through the mouth, the muscle attachments to the chin bone are cut in order to dissect further down on the bone and place the implant.  This allows the implant to migrate upward into the gingivobuccal sulcus.  That sounds like what has occurred.  This is why I prefer using a small, curved incision under the chin to place the chin implant.  This approach preserves the muscle attachment preventing implant migration. You might want to discuss a removal and replacement (using a submental skin incision) with your chin implant surgeon before the implant begins eroding through the inside lining of your mouth.I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  In my opinion, you are a good candidate for chin implant surgery.  
Hope this helps