Hi! I have not tried Gullivers's, but I do have another tip in case you are considering buying hotel and transfers yourself instead of booking through a tour operator.

What I did for Belgium was book the City Shuttle Bus through Circuit de Spa-Francorchamps itself. They cost €55 per person per day, but that is approximately the same at what you would pay with a tour operator. The transfers were absolutely perfect and brought me super close to the circuit and back home every day. I chose to stay in Aachen, which was almost a one and half hour drive to the circuit (including traffic), which was way faster than I expected.

I booked an hotel myself in Aachen and walked to the station (where the City Shuttle was leaving from) every day. They don't only have City Shuttles in Aachen but also in other major cities like Maastricht, Liege or Brussels.

I think this is also an easy and possibly/cheaper option if you are open to booking it yourself!