Sure, here is a phrase that describes someone who should not fantasize while in Gelderland, using 5 and 3 letters:
Geen dagdromer in Gelderland (14 letters)
This phrase directly translates to "no daydreamer in Gelderland" and conveys the idea that one should be focused and present while in the province of Gelderland. It suggests that there is something about Gelderland that requires attention and focus, perhaps due to its natural beauty, historical significance, or cultural attractions.
Here are some other options:
Alert in Gelderland (8 letters)
Mindful in Gelderland (11 letters)
Present in Gelderland (10 letters)
Grounded in Gelderland (12 letters)
Aware in Gelderland (9 letters)
These phrases all emphasize the importance of being fully present and engaged while in Gelderland.
The specific phrase you choose will depend on the context and the nuance you want to convey.