I wouldn't worry about it unless you hook everything up and find that you aren't getting good enough suction. In fact, you don't want to completely seal up the saw, or else there won't be enough air intake to feed the dust collection port. Airflow in must equal the airflow out. If you starve the dust collector by sealing up the table saw too much, you'll only strain the dust collector motor as it tries (and fails) to pull the same volume of air through nothing but the space between your blade and throat plate.
For comparison, I have a cabinet saw with a 4" dust collection port and good dust collection, and just like yours it has gaps on the underside where the cast iron table meets the cabinet. Of course, it also has the big curved hole for the height adjustment crank which allows you to tilt the arbor.
As I mentioned earlier, I wouldn't worry about it for now, but if you do later find that you don't get very good dust collection, you can always try cutting some foam to fill the gaps. Even then, you might be better served by adding dust collection above your blade--for example, some blade guards have dust collection ports.