GM is not gambling with age discrimination lawsuits since to receive the severance pay package of up to 6 months of pay, the person laid off needs to sign an agreement that they won't sue GM. GM has very smartly targeted those older individuals who are very close to reaching 30 years at GM (which allows them to receive their full pensions at 62 instead of 65). If they force this group out prior to 30 years, it will save GM substantial amounts in pension payments.
Why would anybody that close to retirement accept this? If they get laid off without severance they can just sue for age discrimination for much more than six months of pay - and there are plenty of lawyers who would take that case with no upfront payment. So why sign?