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