Gareth Jenkins (in-universe)

Gareth Jenkins assisted the Doctor in defeating the Sontaran Group Marshall Nathan and his confederate.

The Doctor transported Gareth Jenkins from Earth to the TARDIS by accident, Gareth Jenkins had on a boy-sized version of the Doctor's own clothes. He also knew how to operate the TARDIS controls from watching the Doctor do so on Doctor Who and knew what switches to press on it to kill the two Sontarans. The Doctor revealed that in the year 2001, when Sontarans invaded, that the adult Gareth would lead a resistance movement which turns away the invasion fleet. (DW: A Fix with Sontarans)

Behind the Scenes
Gareth Jenkins, showing off his costume, wrote in to Jim'll Fix It to ask if they could let him star in his own episode of Doctor Who. At the end of the mini-episode, Colin Baker presented him with both a Jim'll Fix It medal and the meson gun prop to keep.

A Fix with Sontarans probably does not take place in the continuity of the Doctor Who Universe. Another, unrelated Gareth Jenkins works with Big Finish Productions.