The centre will undergo keyhole surgery on Wednesday to allow medics to see the full extent of the damage, and he is facing a minimum lay-off of four to six weeks.
Ospreys Physio Chris Towers commented: "Gavin's injury hasn't responded to treatment as anticipated and so he was routinely reviewed by an ankle specialist. In light of the ligament damage involved the specialist felt that it was advisable at this stage to examine the potential for any instability of the joint and in this instance, the only way to achieve this is to perform an arthroscopy and screen the ankle in theatre.
"The best case scenario will see Gavin sidelined for a further four to six weeks, with no further action needing to be taken. However, if the examination reveals clinical instability then the plan will be to repair the ligament at that time, which would be likely to sideline Gavin for approximately 12 weeks. It is impossible to say what the outcome will be until the specialist has had a chance to assess the ankle fully, so it would be inappropriate for us to comment further at this moment."
Head Coach Lyn Jones added: "The news comes as a blow for the Ospreys as we look to finish the season in winning style and personally for Gavin, who has been in such good form this year. Hopefully, Wednesday's investigation will bring him good news and he can be back up and about within just a few weeks."