Gap between house and wood deck -- to fill or not to fill?
Part of the reason why a floating deck "works" is that it is not attached to the house in any way. If you put in something that physically connects the deck to the house, even Great Stuff, you will end up with one of two possibilities:


Stays together when the deck moves


If the deck and the house stay together when the deck moves then you will be twisting the deck platform and/or supports, which is not a good thing and could lead to serious problems.


Does NOT stay together when the deck moves


If the deck and the house do not stay together when the moves, then the connection will break - which would mean your time/effort/$ would go to waste.

Floating decks move. That is why they are allowed to be built (in most places, up to a certain size) without a permit and also why they are specifically not to be attached to any fixed structure.

What can you do? I suggest adding a piece of wood trim to fill the gap but attach it only to the deck.