McCormack's husband is suffering from Motor Neurone Disease, making it hard for her to commit to the Quartet’s busy schedule when concerts resume.
She joined the quartet in May 2019, replacing violinist Daniel Rowland.
Violinist Krysia Osostowicz will take the spot, after playing with the Dante String Quartet for 25 years.
"Our time together as a quartet was sadly far too short, but I leave with enough fabulous musical and culinary memories to last me a lifetime," McCormack said.
“We are immensely sad to be losing Gina," the other quartet members said. “Our short time together was hugely productive, rewarding, and immensely fun. Gina has left us with many memories and we shall miss her enormously.
“The current performance restrictions, however, have allowed us the opportunity to consider our future without Gina. We three have decided to continue working together and are relieved and thrilled to have found a new colleague; the wonderful Krysia Osostowicz.
“Having known each other since student days, our similar journeys have converged with serendipity at this new phase in the Brodsky Quartet’s life as we approach our 50th anniversary season.”