Organisers are hoping the black tie event will raise funds for new equipment and infrastructure.
The gym was opened at the end of last month, and took 172 days to build.
Principal Marcus Cooper said he had never known a building to be constructed so quickly.
Mr Cooper said the building was "something [the] school could be proud of".
While it was supposed to service the school and the community for five decades, he had been told it would last for 150 years.
He said the gala would be an opportunity to celebrate the completion of the gym, while also providing a chance to fundraise for gear, including adding to the weights room and buying new sports equipment.
He also hoped to have the cricket pitch resurfaced and practice nets added.
Mr Cooper said the pool and its surroundings also required upgrading.
"Essentially, upgrading the infrastructure of the school to meet the needs of our students and community.
"We envision that the school will act as a community hub for a wide variety of sports and recreational clubs in Geraldine."
Health and physical education head of department Kieran Blogg said he was "very" pleased with his new classroom.
Mr Blogg said there was a long list of bookings for groups interested in using the gym out of school-hours, but there was scope to have more.
"It is a classroom first and foremost," he said.
The main thing for him was that students could train.
The playing area of the gym was constructed to be a passive building, which meant it would not need air conditioning in summer or heat in winter.
However, Mr Blogg said they would not know for sure until winter arrived.
There was also scope to add more solar panels to the roof.
The gala would be held at the gym on Saturday, February 22, beginning at 6.30pm.
Tickets can be purchased online at Humanitix.
