Gunston will miss up to three weeks after injuring the medial ligament in his knee against the Western Bulldogs on Sunday.
Birchall said they believed they could cover the forward, who has kicked 44 goals this year and is third at the club in goal kicking behind Jarrod Roughead and Luke Breust.
"The forward line has been fantastic, there's been a good spread of goal kickers down there and they're functioning really well, so we'll just go about our business," Birchall said on Tuesday.
"It's been a little bit of a change for us. We've copped a fair share of injuries but the guys that have come in have played really well and played their role, so that will continue for the next couple of weeks.
"He's been a fantastic player for us all year and unfortunately he'll miss a couple of weeks.
"It gives him a good opportunity to freshen up.
"He's hardly missed a beat all year so he'll be right to go in a few weeks."
Meanwhile, defender Brian Lake will be available for the Hawks' round 21 clash with Fremantle after serving a month-long suspension for his scuffle with North Melbourne's Drew Petrie in round 16.
He drew criticism for the undisciplined choking action he applied to Petrie as the two players wrestled on the ground.
Birchall said the leadership group would welcome back the Norm Smith medallist next weekend but acknowledged it would be up to the coaches if he gained an immediate call-up.
"That will be up to 'Clarko' and those guys to see what's best for him but from our point of view, he's going really well," he said.
"Brian's a very decorated player, we all know that and when he's at his best he's in our best 22 every day of the week so we'll find out in the next couple of weeks.
"Our fitness coach Andrew Russell has been punishing him quite a bit on the track so I'm sure he'll be nice and fit and ready to go when he comes back.
"Mentally he's fine. Same old Brian. He's going well, he's just really keen to get out there amongst it again and play his role for the team."
The Hawks have suffered multiple changes in defence this season with Ben Stratton starting the season late, Josh Gibson missing eight games and Lake battling a calf problem on and off before his suspension.
Birchall said it would be handy to have their tall backmen available together as the finals approached.
"Our defence this year has been a little bit unsettled I guess," he said.
"But Brian's been going through some solid training at the minute and hopefully we can get him back sooner rather than later because he's really important for the side."
Birchall said Cyril Rioli was "going well" and would look to be available in anywhere between three and four weeks.
