There's probably not a Gold type gym in Oarai as it's a very small town.
All I have been able to find is Yukkurakan, which seems to be a large complex of spas, gym, pool, etc. It's a 15-minute walk from the Oarai Station.
http://www.yukkurakan.jp/
http://www.yukkurakan.jp/sisetu_s.html