Mrs Merkel, who is running for her fourth term in office, now faces limited options after the vote on September 24 after Mr Gabriel dismissed hopes of a grand coalition.
Mr Gabriel, from the Social Democrats (SPD) party – which currently works with Angela Merkel’s party in a coalition – are fielding their own candidate, Martin Schulz, former President of the European Parliament.
But polls predict Mrs Merkel’s ruling coalition of the Christian Democratic Union of Germany (CDU) and its sister party the Christian Social Union in Bavaria (CSU) will win comfortably.
The CSU/CDU is expected to clinch between 37-40 per cent of the vote, with the SPD second on 22-24 per cent.
But ahead of the election, Mr Gabriel revealed he would not support the status quo any longer, and vetoed the idea of being part of Mrs Merkel’s new ‘grand coalition’.
He claimed the reason behind the decision was because the SPD would not be able to nominate a a chancellor as a junior partner.
Previously getting into bed with the CDU/CSU in the 2013 election, and once before in 2005, Mr Gabriel has called time on the SPD’s involvement in ‘grand coalitions’.
He said: “A grand coalition is not sensible because the SPD cannot come up with the chancellor.”
Earlier he cited irreconcilable differences between the parties, following four years of working together.
That leaves Mrs Merkel with several options, a grand coalition remains a possibility if the SPD changes it mind and decides to join, as they did in 2013.
Or she can cobble together a coalition from the remaining parties, dubbed a ‘Jamaica’ coalition, due to the parties colours black, yellow and green.
In a previous election speech where she was heckled, the Chancellor said: “Some believe the problems in Germany can be fixed by screaming – but I don’t think so and the majority of the people here don’t think so either.
“Some people can’t do anything else but scream and shout – but we’re not going to let them lead us astray.”