Chancellor Angela Merkel's conservative bloc (CDU/CSU) and the Social Democratic Party (SPD) agreed a coalition contract after weeks of wrangling - but this will only fly if SPD members approve. The result of their vote is due at the end of the week.
Despite both sides appearing to get a number of their pet policies included, unrest is growing among their membership.
The youth wing of the Social Democrat members rejected the agreement at the weekend - in front of Gabriel and other leaders who had made impassioned appeals for acceptance. Newly elected youth SPD leader Johanna Uekermann said many manifesto policies they had campaigned for, were missing from the agreement.
These included reform of study grants, tax increases to pay for better education and improved regional infrastructure. She appealed for the SPD to talk more with the Greens and the left wing Linke, about possibly forming a minority government.
Gabriel had admitted that the proposed alliance with the CDU/CSU was not exactly what the SPD wanted, but said it was a sensible option. "The rejection of the coalition contract would not increase justice, rather it would reduce it for millions of people in Germany," he said.
A number of younger CDU politicians also made public their distaste for the agreement, and on Sunday published a manifesto "CDU 2017", referring to the date of the next federal election.
The group, which includes a number of national and state-level MPs, is calling for their party to re-think the idea of reducing retirement age to 63 for some people, and is applying pressure for leaders to maintain links to the Free Democratic Party (FDP) - the party's previous coalition partner which failed to enter parliament after September's election.
Leading members of the CDU also on Monday signalled their rejection of the coalition agreement. The chairmen of the party's economics council, the small business association and parliamentary small business circle, all put their names to criticism of the agreement's promises on pensions, minimum wage, energy and tax policies.
The CDU/CSU and the SPD agreed a national €8.50 hourly minimum wage, a reduction of retirement age to 63 for those who had paid into the state system for 45 years and the easing of dual citizenship rules.