I was reading about the skin effect and how the effect increases for higher frequencies. I also read that Litz wires can be used to counteract the effect, but usually only up 1 Mhz frequency. Is there some other way that Ghz frequency circuits or microwave transformers deal with the skin effect?
In the microwave world, the word transformer doesn't usually refer to two coils coupled by a magnetic field.
Instead it's used to refer to resonant waveguide structures that allow coupling signal from a system of one characteristic impedance into a load with a different impedance; or coupled waveguide structures, for example in a microwave balun .
Since the construction of these devices is entirely different from the magnetically coupled coils used at lower frequencies, the parasitic effects and performance limitations are also entirely different. Skin effect does indeed increase the loss of microwave transformers, but no more than it does any other device or transmission line in microwave engineering.