It's been some time since I've used Gparted, but I believe that Gparted still have an option when you edit partition to choose what you want it to be - primary or logical (is what matters to you). If you choose logical, I believe it should be added to extended partition automagically. If not, delete that partition (if that's not a problem?), then create new one (logical) and add it to extended partition manually.
But before you try this post the output of:
as root. What is on that primary partition at the moment?