This is the 4863rd most frequent Dutch word.
gelderse
"Gelderse" refers to something or someone from Gelderland, a province in the Netherlands.
The word 'gelderse' is an adjective here, modifying 'bossen' to specify that the forests are located in Gelderland, a province of the Netherlands.
De Gelderse bossen zijn prachtig in de herfst.
The Gelderland forests are beautiful in autumn.
'Gelders' here is used as a descriptive adjective for the specialty dish, indicating its regional origin from Gelderland.
Ik heb vandaag een stuk gelders stoofpot gegeten.
Today I ate a piece of Gelders stew.
'Gelderse' as a modifying adjective specifies that the traditions being referred to are those from the Gelderland province.