God rest you merry gentlemen

This is an ancient carol familiar to the older generation. I don't think it is used much these days. I have deliberately left the punctuation out and leave you to place the coma.

God rest you, merry gentlemen.  OR
God rest you merry, gentlemen.

The first with the coma after you, indicates 'merry gentlemen.' It might give the picture of a group of gentlemen having a merry, hilarious, wild celebration perhaps after a drinking session! No the adjective 'merry' does not qualify 'gentlemen."

The second is correct. "God rest you merry, gentlemen." "Rest" is old English for "Keep"
"Merry has the connotation of "mighty or strong under difficult circumstances"

So the meaning of the carol is clear. In this world of trials and temptations, ups and downs, may the Lord keep you gentlemen from fear. Let nothing disturb your  equanimity. May you    still have that joy in your hearts.  "Let nothing you dismay" It reflects the message of the angels to the shepherds, "Do not be afraid, I bring you good news of great joy that will be for all the people. Today in the town of David a Saviour has been born to you; he is Christ the Lord."

Notice the ending of each verse, or chorus if you like with the words "Comfort and joy"  "Comfort" comes from the Latin words "With strength" So gentlemen, may the Lord keep you joyful and strong in this trouble world. A lovely Christmas wish or greeting.

God rest ye merry, gentlemen
Let nothing you dismay
Remember, Christ, our Saviour
Was born on Christmas day
To save us all from Satan's power
When we were gone astray
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

In Bethlehem, in Israel,
This blessed Babe was born
And laid within a manger
Upon this blessed morn
The which His Mother Mary
Did nothing take in scorn
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

From God our Heavenly Father
A blessed Angel came;
And unto certain Shepherds
Brought tidings of the same:
How that in Bethlehem was born
The Son of God by Name.
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

"Fear not then," said the Angel,
"Let nothing you affright,
This day is born a Saviour
Of a pure Virgin bright,
To free all those who trust in Him
From Satan's power and might."
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

The shepherds at those tidings
Rejoiced much in mind,
And left their flocks a-feeding
In tempest, storm and wind:
And went to Bethlehem straightway
The Son of God to find.
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

And when they came to Bethlehem
Where our dear Saviour lay,
They found Him in a manger,
Where oxen feed on hay;
His Mother Mary kneeling down,
Unto the Lord did pray.
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy

Now to the Lord sing praises,
All you within this place,
And with true love and brotherhood
Each other now embrace;
This holy tide of Christmas
All other doth deface.
O tidings of comfort and joy,
Comfort and joy
O tidings of comfort and joy.