The moisture pattern
For Grammostola grossa, keep the enclosure mostly dry, with with a water dish and a slight moisture gradient for younger spiders. Use that as a practical pattern across the enclosure instead of trying to make every part of the substrate equally wet.
Watch how quickly the Guarani Giant enclosure changes over several days. The same amount of water behaves differently in a sling tub and a larger adult setup.
Water dish
Keep a clean water dish available to Grammostola grossa once the spider is large enough to use one safely. Refill it before it stays dry for long periods.
Fresh drinking water and substrate moisture serve different purposes in a Guarani Giant enclosure; one should not be used as an excuse to ignore the other.
When the enclosure is too wet
Persistent condensation, a stale smell, widespread mould and substrate that never begins to dry are signs that the Grammostola grossa enclosure may be staying too wet.
If that happens with a Guarani Giant, reduce how much water is added and improve airflow where necessary. Do not solve excess moisture by removing the water dish.
When the enclosure is too dry
If the Grammostola grossa substrate dries through very quickly and the species needs a moisture reserve, add water to a controlled area or use more appropriate substrate depth.
Small Guarani Giants can dry faster than adults, so juvenile enclosures often need more frequent checking even when the adult species is not kept damp.
Seasonal changes
UK heating in winter and warm summer rooms can change how quickly the Grammostola grossa enclosure dries. Adjust watering to the enclosure itself, not to a calendar.
The care target for a Guarani Giant stays the same through the year; what changes is how quickly the room and ventilation move the setup toward drier conditions.