So what is your intention? Swap forks for a different ride? Racing or just riding?
How will you know which set-up is faster?
It might be my personal thought, but I think you're making things more complex for a marginal gain between narrow tired suspension set-up VS wide tire rigid set-up. => choose one.
In any case, have a look at some rigid 29" mtb forks i.e. on aliexpress and put that under the gravel frame if you really want.