For the record: The "GNAT" designation cover both the G7es(TIV) "Falke", G7es(TV) "Zaunkönig" and the G7es(TXI) "Zaunkönig II"** torpedoes...

Note that this is the proper designation of german torpedoes:
Large arabic letter (diametre) - number (lenght) - small arabic letter (type of propulsion/control*) - roman number in parenthesis (the official Kriegsmarine torpedonumber, ie an consecutively running number designated to torpedoes as they were given operational status).

* e = elektrischer Antrieb / s = schall (torpedo mit akustischen Suchkopf)

Some confusion regarding the use of two letters (I think the proper term would be only one letter - in this case just the "s", but you can see both variants used in german records).


** According to some sources, there were only produced 38 examples of the TXI before the end of WW2, and only known use were the 5 torpedoes loaded onto U-534 in april 1945 (it was sunk before having a chance to use the torpedoes). One torpedo made from the salvaged parts (sadly not a very good effort) are on display with the U-534 exibition in UK.

However, I have found a german list from august 1944 stating 88 TXI's in inventory. I assume some were used in combat during the fall/winter.
The main improvement in the TXI compared to the TV was better filters to discriminate the allied acoustic countermeasures that proved to be quite effective against the TIV and TV.