The word gemirá is the future form of gemir in the third person singular.
See the full gemir conjugation

gemir

El mundo traqueteará y gemirá.
The world shall rattle and groan.
El Espíritu Santo en nosotros gemirá, por lo tanto nuestros corazones se sentirán angustiados.
The Holy Spirit in us will also groan, so our heart will feel distressed.
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