real gematrias – such as the ones brought down by the Gra and Baal HaTurim for example, are definitely true. In fact, the gra’s gematrias aren’t limited to simply additive ones; he also uses advanced math like calculus. (He has a gematria for Pi from the words of the pasuk describing the dimensions of the Kiyor in Nach – it’s beyond cool). Furthermore, gematrias play an integral role in kabbalistic systems and concepts. There is a science, rules and axioms – ptretty much lost to us today – governing how gematrias work. However, nowadays, we are clueless thus the ones we invent are nothing more than cute shtick.
As a side note, Rav Henoch Leibowitz ZT”L said that one can say drush as long as the idea or concept being said is true.