Gareth Jenkins (in-universe)

Gareth Jenkins was a kid from Earth who got transported to the TARDIS by accident by the Sixth Doctor shortly after Tegan Jovanka was transported herself. Gareth is quickly observed by the Doctor to wear a boy-sized version of the his own outfit which he claims his nan made for him.

He knew how to operate the TARDIS' controls from having watched the Doctor do so "on the telly". This included knowledge of switches which release a gas lethal to Sontarans.

Sontaran Group Marshal Nathan and Turner recognised him from an event in 2001, when Sontarans attempt to invade Earth, When Gareth lead the Earth defence force which defeated the invasion fleet.

Although the Doctor pointed out that this future, heroic Jenkins could conceivably be a namesake, the Sontarans decided they couldn’t take the risk and tried to kill Gareth anyway, which lead Gareth to press the death switch on the console.

Even though the Sontarans got defeated, the TARDIS alarms go off and they see a fleet of Sontaran ships heading towards them. (HOMEVID: A Fix with Sontarans)

Recalling her unexpected reunion with the Doctor, Tegan thought that if the reunion was a comedy, then Gareth's comic belittling of the Doctor would've been accompanied by comedy stings of incidental music. (PROSE: Fixing a Hole)

Information from invalid sources
Since the ending of A Fix with Sontarans breaks the fourth wall so thoroughly, the Wiki does not consider it a valid source, and consequently did not recognise its events (and the in-universe Gareth Jenkins) to be part of the Doctor Who universe as we define it, until the story was edited to change its ending to that of several Sontaran ships hunting the TARDIS.