What's the earliest use of the word "gematria" followed by the use of it to mean the value of the word? I'm aware that in Pirkei Avot the word itself is used, but that is not used in conjunction with the actual numbering system (and thus it is possible that it means something else in that context). There are also places in the Gemara that use the value system, but in those places it does not use the word "gematria" but rather "sifra" (lit. "the value"). When is the first time the two are used together?
Related (but not the same): What is the earliest source that mentions Gematria?