Mr Gray and Mr McClymont will make the case for Labour’s plans to extend devolution in the cross party talks, which now have a full line-up that includes former SNP, Conservative and Liberal Democrat leaders.
Finance secretary John Swinney, who led the SNP from 2000 to 2004, and SNP MSP Linda Fabiani have already been appointed to act on behalf of the Nationalists in the talks, which will take place as part of the work of the devolution commission chaired by Lord Smith of Kelvin.
Labour finance spokesman Mr Gray has been formally handed the role, which will see him pitted against Mr Swinney in the talks over a devolution deal that was pledged by the main Unionist parties during the referendum campaign.
Mr McClymont, the MP for Cumbernauld, Kilsyth and Kirkintilloch East, who is currently overseas, is expected to be confirmed as a member of the commission in the coming days.
The shadow UK minister served on Scottish Labour leader Johann Lamont’s devolution group, which recommended giving Scotland control of 15 pence out of the 20 pence basic rate, and limited Holyrood’s control over taxing the highest earners.
Ms Lamont said: “The people of Scotland voted for faster, better, safer change and all the parties now agree that is what they will get.
“We set out a package of proposals which we believe will strengthen the Scottish Parliament while retaining the benefits of the UK. We will engage in the Smith Commission constructively and positively to find a consensus but are determined to ensure Scotland has the best of both worlds.”
The appointment of Mr Gray and Mr McClymont comes just days after the pro-federalist Lib Dems named former Scottish party leader Tavish Scott and Michael Moore, who was Scottish secretary until last year, as their negotiators.
Former Scottish Conservative leader Annabel Goldie and Adam Tomkins, a professor of public law at Glasgow University, have been selected to lead for their party, which has backed a plan to devolve the bulk of income tax powers to Holyrood.
The pro-independence Scottish Greens will be represented by the party’s co-leader Patrick Harvie and Edinburgh councillor Maggie Chapman.
An opening round of negotiations is expected in the next few weeks, as part of a timetable for delivering a radical package of further devolution that will see a “Command Paper” to guide the process published by the end of October and an outline agreement at the end of November.
Lord Smith is set to receive formal submissions from the parties by the end of October, as well as representations from civic institutions and the public.
Formal talks are also set to take place, when the parties will each make the case for their positions on extending devolution.
The Scottish Lib Dems are the only major party to back federalism, which would create a series of regional and national parliaments and assemblies across the UK, with a federal government retaining powers over foreign affairs, defence, currency, welfare and pensions.
Scottish Liberal Democrat leader Willie Rennie yesterday called for the package of new powers pledged to Holyrood to be used to help businesses create jobs.