Bale, who scored with a seventh-minute header to put Real 1-0 up at the Bernabeu, limped out shortly before halftime with his side leading 5-0.
"After tests carried out today on the player Gareth Bale at the Sanitas La Moraleja University Hospital, he has been diagnosed with a soleus muscle injury in his right leg," Real said on their website (www.realmadrid.com).
They did not say how long the Wales international would be sidelined. Spanish media reported he would be out for around three weeks and was targeting a return in the Champions League last 16, first leg at AS Roma on Feb. 17.
Bale had already been sidelined twice this season with calf problems, missing about two weeks each time.
The 26-year-old has been in good form in recent weeks and his goal against Gijon was his 13th in the league this season.
(Reporting by Iain Rogers, editing by Ed Osmond)
