what is grammatical equivalence
It seems to have to do with translation:

In translation terms, equivalence is a term used to refer to the nature and extent of the relationships between SL (source language) and TL (target language) texts or smaller linguistic units. Grammatical equivalence refers to the diversity of grammatical categories across languages. ...Grammatical rules may vary across languages and this may pose some problems in terms of finding a direct correspondence in the TL. In fact, different grammatical structures in the SL and TL may cause remarkable changes in the way the information or message is carried across. These changes may induce the translator either to add or to omit information in the TT because of the lack of particular grammatical devices in the TL itself.
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