You have 3 choices.
1) Don't upgrade to/install Mojave until Nvidia release Mojave-compatible video drivers for unsupported GPUs.
2) If you already have or are going to upgrade to Mojave, remove any non-natively supported Nvidia GPUs and plug your monitor into one of your MB's native Intel graphics ports. Or...
3) Purchase one of the Mojave natively-supported AMD GPUs.