This is the common salutation in all Paul’s Epistles, only in one or two mercy is also added.
Grace signifies free love.
Peace signifies either a reconciliation with God, or brotherly love and unity each with other: See Poole on "Rom_1:7". The apostle wisheth them spiritual blessings, and the greatest spiritual blessings, grace and peace, and that not from and with men, but from God our Father, and theLord Jesus Christ.