The star-studded hosts were grateful for Gillet's effort on the stroke of half-time after Ivan Perisic struck for the visitors after just six minutes.
The result ensured both sides maintained their unbeaten starts but Belgium may be the more disappointed after failing to make the most of opportunities to kill off one of their chief rivals.
Croatia goalkeeper Stipe Pletikosa was kept busy throughout, particularly by the influential Dries Mertens, whose delivery from wide was dangerous.
Pletikosa was first in action after four minutes when he kept out an effort from Zenit St Petersburg's £32million man Axel Witsel.
Croatia went ahead soon after when Perisic struck with a low shot from inside the area.
Belgium tried to respond with Mertens getting forward but Pletikosa pushed away one ball in and came out to claim another.
Christian Benteke shot over and Witsel also headed above the bar in the closing minutes of the first half.
Their equaliser came in injury time as Gillet tried his luck from distance and beat Pletikosa.
The goalkeeper was then in action again early in the second half to keep out a Benteke shot and to punch away a Mertens corner.
Witsel also went close before Belgium goalkeeper Thibaut Courtois was tested at the other end by a Darijo Srna free-kick.
Belgium substitute Kevin Mirallas, the Everton striker, had a late chance for a winner but shot over.