Reap, glean, gather, garner, harvest are comparable when they mean to do the work or a given part of the work of collecting ripened crops.
Reap applies to the cutting down and usually collecting of ripened grain; in extension, it may suggest a return or requital.
Glean basically applies to the stripping of a field or vine that has already been gone over once but may be extended to any picking up of valuable bits from here and there and especially to a gathering of what has been left or missed.
Gather, the most general of these terms, applies to the collecting or bringing together of the produce of the farm, plantation, or garden; in extension, it can apply to any similar amassing or accumulating.
Garner implies the storing of produce (as grain); in extension, it can apply to a laying away of a store.
Harvest, the general term, may imply any or all of these processes or be extended in meaning to apply to any gathering together or husbanding.