From American Memory:

How the gates came ajar [song sheet] (By H. L. Bostwick. J. H. Johnson, Song Publisher &c., No. 7 North 10th. Street, Philadelphia, [n. d.])

No tune given or indicated, but it has a chorus as:

CHO.--O, angel, sweet angel, I pray you,
Let the beautiful gates ajar,
Only a little, I pray you,
Let the beautiful gates ajar.

Other hymns and songs with that phrase at American Memory are:

Gone within the gates ajar / by L. O. Emerson

Golden gates ajar! / by J. L. Feeney

The Glorious gates ajar! / by S. B. Whitely

Passed within the gates ajar / by Charles H. Gabriel

The Pearly Gates ajar / by W. O. Perkins

He holds the pearly gates ajar / by J. Calvin Bushey