The 19-year-old Italian Antonelli lapped the Spa-Francorchamps circuit with a best time of one minute 45.944 seconds in practice two, 0.190 quicker than McLaren's reigning world champion Lando Norris.
Australia's 2025 winner Oscar Piastri was fifth in the earlier run and then sixth after a hydraulic leak at the end of practice one cost him 19 minutes of the later session while the team changed his car's gearbox.
"Our pace is looking reasonable. We are heading in the right direction, and we are in a good place to build on that in FP3 tomorrow and into qualifying," the 25-year-old Melburnian said.
His team-mate Norris was on pole position at Spa last year but this time will have a 10-place grid penalty added to wherever he qualifies on Saturday.
"We've not really changed anything this weekend, no reason for us to expect anything different," said Norris after second free practice.
"I think we're still probably the fourth fastest. Seems like if we can get things right then we might be a little bit closer than we were in Silverstone (last week)."
Red Bull's Max Verstappen was third on the later timesheets, 0.472 off Antonelli's time, after leading the way in first practice with a time of 1:47.070.
Gasly crashed his Alpine at Stavelot with 14 minutes remaining, smashing his car's rear wing and suspension and bringing out the red flags.
"Just absolutely lost it out of nowhere," the Frenchman said over the team radio. "I must have lost the rear."
By the time the track was clear again, there was not enough time for more quick laps with drivers focusing on practice starts.
Ferrari's Lewis Hamilton -- chasing a record-equalling sixth Belgian victory -- was second fastest in the opening session, 0.145 slower than Verstappen, and fourth later on.
Belgian-born four-time champion Verstappen has won three times at Spa-Francorchamps, most recently in 2023.
The Dutch driver has crashed at speed in the last two races, accidents that have led Red Bull to revert to a more conventional rear wing.
Team-mate Isack Hadjar, who will start at the back on Sunday due to engine penalties, was fourth and fifth in the two sessions behind Piastri.
Antonelli had fumed at Williams' Carlos Sainz in the first session, calling the Spaniard an idiot for blocking him.
Mercedes' George Russell was eighth in both sessions, ahead of Racing Bulls rookie Arvid Lindblad.
Russell is 25 points behind Antonelli after nine rounds of the championship with Piastri sixth on 82 points.
