De or het gres?

The correct Dutch article is:

het gres

Meaning (Dutch) 1. dichtgesinterd aardewerk van vette klei 2. Belgische kiezelzandsteen

How to use gres

the greshet gres
this gresdit gres
that gresdat gres
our gresons gres
your gresjouw gres
every greselk gres
which gres?welk gres?
a big gres / the big greseen groot gres, het grote gres

Dutch words ending in gres

regres | congres | partijcongres | ledencongres | jongerencongres | aidscongres

In compound words, the last part decides the article. See the rule

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).