The transaction, which is due to complete on 28 March, does not include the Manchester-based local TV station Channel M and two local newspapers in Woking.
GMG is receiving £7.4m in cash in the deal, with the remaining £37.4m value comes from Trinity Mirror releasing GMG from a long-term contract for printing MEN Media newspapers at its Oldham press.
The deal gives Trinity Mirror control of the MEN Media division, consisting of flagship newspaper the Manchester Evening News and free daily Metro, as well as 20 other papers in the North West of England.
Metro is produced in Manchester under a joint venture between Associated Newspapers and GMG. Trinity will now take GMG's stake in that venture.
The deal also covers S&B Media, which publishes 10 titles in the South of England, including the Surrey Advertiser and the Reading Post.
The acquisition will result in a number of executive changes, including the departures of GMG Regional Media chief executive Mark Dodson and MEN Media managing director Ruth Spratt.
GMG has appointed David Sharrock, currently chief operating officer of GMG Regional Media, as managing director of MEN Media.
Following the completion of the deal, S&B Media will be managed as part of Trinity Mirror's existing publishing business in the South of England.
A GMG spokesman refused to comment on whether closing Channel M was an option, ahead of a meeting called for the station's approximately 30 staff at midday.
The spokesman said GMG intended to find another buyer for the two Woking papers it still owns.
In a statement, Trinity Mirror chief executive Sly Bailey said GMG Regional Media was "a perfect strategic fit".
While Bailey praised the "proud and rich journalistic heritage" of the Manchester Evening News, GMG chief executive Carolyn McCall said the MEN and its sister titles "have made a huge contribution to the fortunes of the Group for the best part of a century".
However, McCall went on to say the decision to sell was made to secure the future of GMG's flagship national newspaper The Guardian.
"GMG is mandated to secure the future of the Guardian in perpetuity, and we have a strong portfolio which has to be in the right shape to achieve that goal."
In GMG's last financial year to the end of March 2009, GMG Regional Media's operating profit was £500,000, down from £14.3m in 2008.
The disposal means the group, which has its roots in the founding of the Manchester Guardian in 1821, will sever its ties with local newspaper publishing in Manchester.
The Manchester Evening News and The Guardian, which was known as The Manchester Guardian until 1959, have been under the same ownership since 1924.
In November Trinity Mirror reported that ad spend earned by its national papers would be down just 5% in November, while ad revenue for its regional papers would be down around 22%.
Though a big drop, it was an improvement from earlier in the year. Trinity Mirror's regional arm posted a 35% drop in ad revenue in the first half of 2009.
Trinity has spent much of the past 18 months reviewing its regional operations, resulting in the closure of a number of papers and radical changes to others.
In October it announced the Birmingham Post would go weekly and its afternoon Midlands paper, the Birmingham Mail, would switch to mornings. 80 job losses were expected as a result of the changes.