If this is just an injury from your toothbrush, it's not unlikely for your gum to grow back. The key is to go to your periodontist to make sure that it is not also reflective of any other condition as well as to review proper home care.
Please don't worry yourself
Looks like a simple traumatic injury that should heal just fine by avoiding the aggressive brushing you were subjecting it to with the hard brush. Once completely healed and healthy, you may want to actually have this gum cosmetically contoured to remove some of the gum and make it more similar in height and contour to the other pre-molar which is in front of it.
Douglas Jopling, DDS
Dallas, TX area
The gum may not grow back
There are many factors at play here. Usually if the gum has receded by itself it will not come back without the help of a graft procedure. If it's an acute trauma that involves bleeding, it may grow back on its own. Definitely see your dentist as soon as possible before it worsens.
This looks like a traumatic injury. Easy to fix with a gum graft procedure. See your local periodontist and all the best
A Chunk of Gum Missing
Gums are delicate structures. And they respond very positively to good brushing habits and cleanliness. So yes, once you stop the traumatic brushing and flossing you they will come back. Especially at age 18! First: stop brushing your teeth! Yes I mean that. Start massaging your gums will a super soft toothbrush that has been warmed up in warm/hot water. This gets the bristles really soft. Then, at a 45 degree angle into the gums, gently massage the gums around that and all your teeth with just a little toothpaste (not like what you see on TV. Much, much less is needed.) For flossing two things: make sure you hug the tooth by forming a complete sideways "C" along the side of the tooth. Go to just gum height. Do not take it down in to the gum as that is where gum cut's are made. Be gentle. Your gum massaging will do the rest. The final act is to put your bristles on the outside of your gums and pull the gum tissue down on to your tooth. Use motion to tell that gum where to go. It will heal as you would like it to heal this way. Let me know if you have any other questions.
one of the reasons for gum recession is agressive brushing and using hard tooth brush. Other causes include orthodontic treatment ( braces), bite problems, smoking ...
It is not very clear in your oicture ,but I think I see a cut in the gum as well. This is a gingival cleft that could be caused by wrong method of flossing and/ or bite problems on that specific tooth.
Use a soft tooth brush from now on. And see a dentist to evaluate the situation. Recession will not correct itself. You might need to be referred for treatment.