Thats quite a gap
But its pretty simple to resolve i think going by the photo's.
Do it all as a dry fit to start with, if you can make it fit dry u can do it again with glue.
Put the deck on from the stern (rear of the boat at transom) hooking on with some tape to make sure it does not pull off, now gently pull the deck to the bow,(front of the boat) if the kits made well it should be a tight fit but will slide together. (the boat will flex better to the bow so that few mm gap will be countered by the inward force of the hull being squeezed from stern to bow, and the hull walls will push out this will not only fill that gap but also make the boat a tad wider once the decks in place.)
Now if that all goes well remove and apply the right glue for the job (I tend to use a resin,30 min araldite for deck joining regardless of material GPR,Stiro etc, as this has a enough work time but yet drys quickly) then put the deck back on in the same way you dry fitted it, clamp it together with lots of rubber bands until the glue sets.
If it still
does not fit, don't glue it!!, as there may be a problem with the moulding which you may need to address with manufacturer. there are other methods using fillers etc, to fill gaps but i think in this case its not needed as the moulding should be pretty accurate or what was the point of making a kit that dose not fit together correctly....