The last version I tried, just a few months ago ... it simply crashed in the middle of the partitioning.
It did not complete, or cause any errors.... it is never good to have a partitioning app crash in the middle of its job.
Back with 5.4, although gparted never crashed, the sabayon (fedora) installer, still did not like the new partition scheme, fdisk was the only way I could get past the installer crashing.
The little research I have done, appears this never happens on fedora, is a bug in the sabayon code that only affects a few people, a few machines.
But enough that on irc #sabayon I see it often.
I think that using fdisk a command line tool to create and delete do partition work etc..... can be a little overwhelming for some.
A:, it has never crashed on me
B:, it only does exactly what I tell it to do
c:, it is pretty easy to use
d:. has a great help section
And if we are simply doing a sabayon install single boot, use the whole drive. My above instructions can be completed with the tools provided on sabayon, and done in a min. often one can continue on with the installer without rebooting.
I am not a fanboi command line junkie, but fdisk is certainly a tool that I prefer to use.
If one is going to use gparted to create the partitions, I would still advise to use gparted to simply delete them in this case.
Use the installer to create the new partitions.
In the case above, op already tried gparted, got poor results. I will admit that gparted will work for 90% of users with no issues.
For the rest, just fdisk and move on.