Gareth is a Welsh Under 21 international left back who enjoyed a magnificent season with the Peterborough Under 19s in 1998/99. Sparkling performances led to a stint in the Reserves towards the back end of last term after returning from a successful loan spell with Dr. Martens outfit Boston United. Plucked from Norwich City at 12 years of age by Posh Academy Director Kit Carson, he is a player with plenty of pace and strength in the tackle who likes to get forward. Made 20 appearances in 1999/200 leading to a new five year contract. However injuries limited him to 8 appearances in 2000/01.

In August 2004, Gareth joined the Pilgrims of Boston United on loan. After returning to London Road, Gareth was allowed to sign for Mansfield following Peterborough's re-signing of Mark Arber. He made his debut in Mansfield's draw with Southend on 22nd January 2005.

Despite making 45 appearances during 2007/8, scoring once, Jelleyman could not help Mansfield avoid relegation out of the Football League and into the Conference

Released by Mansfield, Jelleyman trialled with Rushden & Diamonds in July 2008 and then signed a one year contract with them on 21st July 2008

In March 2009, Gareth joined Barrow on loan. After joining Telford on non-contract terms, Jelleyman signed for Barrow on 28th August 2009

On 12th May 2010, Barrow released winger Carlos Logan, striker Luke Powell along with Jelleyman who had made 26 appearances for the Bluebirds. He signed for Boston United on 30th July 2010. United boss Rob Scott said: “It is quite a coup to land a player of Gareth’s quality. We have bided our time and are delighted to welcome Gareth onboard.”

On 22nd February 2013 Jelleyman was released by Boston United and he subsequently signed for Corby Town.