"Het gewas is klein."
Translation:The crop is small.
Harvest = oogst. Crop can also mean oogst.
However, gewas can mean crop, but not harvest.
As you can see crop has multiple definitions in English. Gewas does not meet all those definitions. That would be a very simplistic approach.
Check the meanings available on the OED (I don't know how to take a screenshot with my mobile, sorry, and that's the only device where I have it -I cannot copy/paste from the app) and then you'll see what I meant.
And precisely, if you analyse the meanings you propose (#2, first link and #4, second link), you'll notice that they are not exactly the same. What is more: Meaning number 4 on the second link says that "harvest" means "gathered crops", while normally "crop" means plants that were gown as to be harvested.
Granted: right at the bottom they mention that they can be used as synonyms, but nevertheless there are subtle differences.
On the other hand, if you check on http://www.macmillandictionary.com/thesaurus-category/british/crop-farming under harvest you'll find the following meanings:
harvest (noun): the activity of collecting a crop
harvest (noun): the time when a crop is collected
harvest (noun): the amount of a crop that is collected
harvest (verb): to collect a crop from the fields
Crop is defined as:
"a (1) : a plant or animal or plant or animal product that can be grown and harvested extensively for profit or subsistence an apple crop a crop of wool
(2) : the total yearly production from a specified area
b : the product or yield of something formed together the ice crop
c : a batch or lot of something produced during a particular cycle the current crop of films
d : collection a crop of lies"
While harvest is defined as:
" 1 : the season for gathering in agricultural crops the beginning of the harvest<pre>
2 : the act or process of gathering in a crop assisting neighbors in their harvest 3 a : a mature crop (as of grain or fruit) : yield bountiful harvests b : the quantity of a natural product gathered in a single season the salmon harvest timber harvests 4: an accumulated store or productive result a harvest of revenue"</pre>
So, there is a slight difference: a crop is still on the land, while a harvest is not, since it's been harvested (as redundant as that may sound).