It sounds like you only checked the volumes of GM that go through 1635. The Great Migration Directory includes individuals who came between 1636 and 1640, and states:
John Leonard: Unknown; 1638; Springfield [SpTR 161; SpVR 19; Pynchon Court 205 ; Hale-House 678-82].
Since he is in the GMD, he definitely gets the Puritan Great Migration project box, so I have added it to his profile.
The unknown means Anderson views his origin as unknown. If you want to update the profile with more sources, the codes here mean:
SpTR: Springfield, Massachusetts, Town Records
SpVR: Clifford L. Stott, comp., Vital Records of Springfield, Massachusetts to 1850, 6 vols. (Boston 2003)
Pynchon Court: Colonial Justice in Western Massachusetts (1639-1702), The Pynchon Court Record. An Original Judges’ Diary of the Administration of Justice in the Springfield Courts in the Massachusetts Bay Colony, Joseph H. Smith, ed. (Cambridge, Massachusetts, 1961)
Hale-House: Donald Lines Jacobus and Edgar Francis Waterman, Hale, House and Related Families, Mainly of the Connecticut River Valley (Hartford 1952; rpt. Baltimore 1978)