in \Slicer4\Modules\Loadable\VolumeRendering\qSlicerVolumeRenderingSettingsPanel.cxx

REPLACE

if (gpuInfoList->GetNumberOfGPUs() > 0)
{
int gpuMemoryInBytes = gpuInfoList->GetGPUInfo(0)->GetDedicatedVideoMemory();
int gpuMemoryInKo = gpuMemoryInBytes / 1024;
int gpuMemoryInMo = gpuMemoryInKo / 1024;
// Set it as the default amount of memory
q->setGPUMemory(gpuMemoryInMo);
}

q->registerProperty("VolumeRendering/GPUMemorySize", q, "gpuMemory",
SIGNAL(gpuMemoryChanged(int)));
}

WITH

if (gpuInfoList->GetNumberOfGPUs() > 0)
{
// double gpuMemoryInBytes = gpuInfoList->GetGPUInfo(0)->GetDedicatedVideoMemory();
// double gpuMemoryInKo = gpuMemoryInBytes / 1024;
// double gpuMemoryInMo =
// Set it as the default amount of memory
q->setGPUMemory(gpuInfoList->GetGPUInfo(0)->GetDedicatedVideoMemory()>>20);
}

q->registerProperty("VolumeRendering/GPUMemorySize", q, "gpuMemory",
SIGNAL(gpuMemoryChanged(int)));
}