Can an Injury to the Pec Area Trigger Gynecomastia?

Your description is quite accurate for a partial muscle tear causing an intramuscular hematoma (which is what caused the bruising and swelling). This should have been diagnosed and evacuated at the time of the injury, but letting this collection of blood reabsorb over 6 months' time has left persistent scar tissue in the right pec area.

Shame on your "cosmetic surgeon" consultant for failing to diagnose this, though 6 months after injury this may well have looked less like an old collection of blood and more like a fibrous mass of scar tissue that closely mimicked the consistency of gynecomastia tissue. Damage to the pec minor would have been less likely than damage to the pec major, but whichever muscle was injured is immaterial to the diagnosis.

At this point you require excision of the scar mass to equalize the chest/breast area. This can be done through a short periareolar incision, but beware--liposuction (including ultrasonic and laser-assisted) will be a spectacular failure and will NOT make any real improvement!  This scar tissue is very firm and fibrous--not at all like fatty tissue--and totally inappropriate for liposuction. Proper surgical treatment of this will require subcutaneous excision of the excess bulk, preserving a small disc of normal tissue beneath the nipple areola complex to prevent unnatural adherence to the underlying muscle, and placement of a drain for about a week. Make sure you see an American Board of Plastic Surgery-certified plastic surgeon, not a "cosmetic surgeon" who may have little true general surgical or plastic surgical training or experience!

It sounds as though you may have ruptured part of your chest muscles.  They then balled up can are causing your gynecomastia.  The treatment may involve slightly larger scars, but the excess muscle may be amenable to excision in order to improve the appearance of your chest.

Even when a muscle tears and causes a deformity (the most obvious one is biceps tear), unless it is immediately repaired, it can't be repaired.  What you can do is work on the soft tissue to disguise it.  Liposuction or excision of some breast tissue can be part of the cure.  Fat transfer or custom made implants for indentations.