The two companies recently offered to sell four brands of Scotch - but not their top-selling J&B, Johnnie Walker or Dewar's lines - to satisfy the Commission's objections, according to sources familiar with the case.
The offer suggests GrandMet and Guinness are going to fight to retain their most prominent Scotch brands, even though competitors argue that concentrating those leading brands in a single company would mean higher prices for consumers.
A spokesman for the Commission said the executive agency of the European Union will be able to conclude its five-month investigation into the planned merger ahead of the 27 October deadline.
"We have now done our homework," he said, declining to add what undertakings the companies had offered. All that remained were "some further technical discussions" before the ruling.
Guinness and GrandMet officials refused to comment on the EU review. A spokesman for the companies in London said he had not been told officially that the ruling would come on 15 October. "Until a judgment is handed down by the people in power, we aren't going to say a word," he said.
In August, the Commission said the combined company would have too much control of the spirits market in Greece, Spain, Ireland, Belgium, Luxembourg and Britain. The Commission said the new company formed by the merger, GMG Brands, could thwart competition for three products - Scotch whisky, gin and vodka.
The GrandMet-Guinness proposal would attempt to address those charges by selling the rights to four brands - Ainslie's, Haig, White Horse and Vat 69 - throughout Europe. The proposal also offers other concessions on a country-by-country basis, according to a source close to the case.
While the Commission rarely blocks mergers or acquisitions, it can force the companies involved to sell off assets in order to reduce their market share in specific countries or of specific products.
The merger also needs anti-trust approval from the US Federal Trade Commission, which is expected to make a decision this autumn. GMG faces similar competition issues in the US. Although the company would have only about 25 per cent of the US distilled spirits market, it would have 60 per cent of the standard Scotch market, according to industry research group Impact International.
Analysts predict that GMG will probably have to sell the US rights to some product lines as well, possibly including the Guinness Dewar's brand, which has a high sales volume but a low profit margin.
The proposed merger also has come under fire from Bernard Arnault, chairman of the French drinks and luxury goods company LVMH.
Mr Arnault is seeking to have his company, which owns more than 10 per cent of the stock of GrandMet and Guinness, included in the proposed combination.
Spirits rivals Seagram and Allied Domecq are also opposing the merger.