You will need to use UART1 for your GPS, or a softserial port… UART6 (RX6 and TX6) is used by the sbus input from your receiver.
Part of the fun with the omnibus baurds is that there are so many versions, alot of the video are out of date… the first vew versions SBUS was on UART1, which is why GPS is connected to UART6, but with V5 (and V4 i think) SBUS is now on UART6, so you need to hook up the GPS to UART6… or soft serial.
Using soft serial is explained here:
Also I beleive most Ublox modules require 5V, not 3v3 so that could also contribute to your problem. If you are not using SBUS receiver I think you can still use UART6 for your GPS (but I have never tested as I always use SBUS RX recently)