Strictly speaking, the counterpart of Grantha "+" is not European "+", but the "straight double quotation mark" (so called "dicto sign") or the "points of ellipsis" ("..."), the latter especially in the way they are used in mathematics, where they mean "and so forth, up to" ("1, 2, ..., 10").
On the other hand, the Grantha "+" can be interpreted as if meaning "plus (or) add [text as already written previously]", and thus it can be easily connected with the European "+".
In Europe the "+" sign came into use in the 14th century, as an abbreviation for Latin "et", "and"; it was used for addition since the 15th century. Grantha manuscripts are not older than the XVII century, so they aren't of great help here. One should look into Grantha inscriptions earlier than the 14th century, and see if the "+" sign was already used there.
Best,
Marco
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