Gareth Jenkins (in-universe)

In the 1980s, Gareth Jenkins was a child who wrote into the BBC One audience wish-fulfilment programme, Jim'll Fix It, to ask if they could let him star in his own episode of Doctor Who. As a part of the request, he demonstrated that he already had a complete Sixth Doctor's costume, which had been made for him by his grandmother.

The producers of Jim'll Fix It agreed, and worked with John Nathan-Turner's staff at Doctor Who to create the sketch, A Fix with Sontarans. Jenkins — and his kid-sized Sixth Doctor's costume — did indeed play a major role in it, opposite Colin Baker and Janet Fielding. At the conclusion of the episode, Baker presented Jenkins with both a Jim'll Fix It medal and the "meson gun" prop to keep.

Jenkins reunited with Colin Baker at a signing event in January 2013.

He now works as head of campaigns for the Save the Children charity. He still has the medal. In 2015, he was interviewed by Toby Hadoke for Round 107 of Toby Hadoke's Who's Round. He also contributed to the charity reference book Behind the Sofa: Celebrity Memories of Doctor Who.

Gareth Jenkins, the "character"
Since A Fix with Sontarans's ending so thoroughly breaks the fourth wall, the Wiki does not consider it a valid source, and consequently does not recognise its events (and the in-universe Gareth Jenkins) to be part of the Doctor Who universe as we define it.

Within the narrative segment of A Fix with Sontarans, Jenkins gets transported from Earth to the Doctor's TARDIS by accident, and is observed by the Doctor to wear a boy-sized version of the Doctor's outfit; he knows how to operate the TARDIS' controls from watching the Doctor do so from having watched the Doctor on telly. This includes knowledge of switches which release a gas lethal to Sontarans, interestingly a discrepancy between the in-universe Doctor Who series and the real programme, as this feature of the TARDIS control console was never seen in the TV series proper.

Further detail is given about the in-universe Jenkins when Group Marshall Nathan explains that in the year 2001, when Sontarans attempt to invade the Earth, a man called Gareth Jenkins wil lead the Earth defence force which defeats the invasion fleet. The Doctor points out that this future, heroic Jenkins could conceivably be a namesake, though the Sontarans decide they can't take the risk and try to kill Gareth anyway, hence his pressing the aforementioned death-switch on the console.