These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.22 s (backtrack: 1/20).



[nomerge       ] dev-lang/mono-6.12.0.199-r2::gentoo  USE="nls -doc -minimal -pax-kernel (-selinux) -xen" 
[ebuild  N     ]  dev-dotnet/libgdiplus-6.2::gentoo  USE="cairo" 2231 KiB
[ebuild  N     ]   dev-lang/mono-6.12.0.199-r2::gentoo  USE="nls -doc -minimal -pax-kernel (-selinux) -xen" 297019 KiB

Total: 2 packages (2 new), Size of downloads: 299249 KiB

 * Error: circular dependencies:

(dev-dotnet/libgdiplus-6.2:0/0::gentoo, ebuild scheduled for merge) depends on
 (dev-lang/mono-6.12.0.199-r2:0/0::gentoo, ebuild scheduled for merge) (buildtime)
  (dev-dotnet/libgdiplus-6.2:0/0::gentoo, ebuild scheduled for merge) (buildtime, USE=-minimal)

It might be possible to break this cycle
by applying the following change:
- dev-lang/mono-6.12.0.199-r2 (Change USE: +minimal)

Note that this change can be reverted, once the package has been installed.