Grinstead is currently strategic development and marketing director at the firm, while Gardner has been in the post of chief executive since early 2008.
Metlife says the move is part of a restructuring of its European business and that Gardner will be leaving the business at the end of June.
In a statement the firm says: “Metlife Europe is reorganising its operational structure in order to increase momentum in the UK and to deliver ambitious growth plans in continental Europe.
“The reorganisation is further evidence of Metlife’s sales success and financial strength which enables it to deliver strong and credible guarantees no matter what the market conditions are.
“The new structure will mean Metlife Europe expanding its remit to cover existing international businesses and future launches.”
Dominic Grinstead joined Metlife in 2005 and has previously held a number of senior roles including head of national sales at Clerical Medical.
Metlife says Gardner has played “a key role in the success of the business over the past 18 months”.