How much guru charges it’s fees per project ?
Who/what/when/where/why is Guru? I hope you are not referring to another freelancing platform as that’s not allowed here on the forum.
If you are referring to a particular seller, you can check their gig/contact them to find out how much they charge.
Is the rule about mentioning competing platforms mentioned anywhere (like in the TOS or in one of the community rules sections)?
Not that I know of. No. All I know is that it is not allowed and that I have seen posts get flagged by mods/the community when someone does that. It’s probably one of those “unsaid rules.”