Downloaded Z77NWIFI.F3-XMAX.rom from here - thanks so much!
I was able to install Mojave without issues.
I have 3 issues on GA-Z77N-WIFI with Mojave macOS 10.14
1) I have 2x4GB total 8GB (BIOS shows correct size) RAM is showing 4GB in macOS Mojave. > Fixed (sudo nvram 1F8E0C02-58A9-4E34-AE22-2B63745FA101:SkipSmbusSpdScan=%01)
2) I have ATI Radeon HD 5770 resolution is only 1280 x 10243) 2 Ethernet ports are not being recognized > (FIXED by downloading and installing RealtekRTL81xx-0_0_90.pkg here)
I would appreciate if anyone has any idea how to fix 3 issue above, thanks for your help in advance.