Fill drained rocks or gravels around tree base surrounding by deck
The tree will continue to expand as it grows. If you completely fill that space, it may start to push the rocks against the deck.
More important, whatever you do make sure there's plenty of drainage. The trunk will be at risk of rot damage if you surround it with materials that hold water -- and remember that it only takes a couple years for dust &amp; dirt to fill in the gaps in coarse gravel.
Personally, both for the tree's health and for aesthetics, I'd leave the area open.