When you get that message, it means SYSLINUX can't find the configuration file, syslinux.cfg (or it is indeed not written properly - unlikely in your case). For GPartedLive, it is located in /syslinux.
A quick workaround is to copy the contents of /syslinux (everything in it) to the root of your USB drive. SYSLINUX will always look in the root for syslinux.cfg. SYSLINUX 3.86 also seems to have some bugs with its ability to find the configuration file when it is nested inside directories - SYSLINUX 4.0 seems a lot more reliable and resilient.
For a not-so-quick-and-less-likely-to-work-but-potentially-cleaner workaround, try manually telling SYSLINUX where to look by doing the following (assuming you're using Windows) -
- Download SYSLINUX 3.86.
- Extract win32\syslinux.exe to your desktop.
- Run the Command Prompt as administrator (Start -> All Programs -> Accessories -> right-click on Command Prompt, click Run as administrator).
- Find your desktop, if Windows Vista/7, usually, cd \users\username\Desktop, replacing username with your Windows username.
- Type syslinux.exe -fma -d /syslinux H: replacing H: with the letter of your USB drive. Be careful - do not replace H: with C: or whatever your Windows drive is, or you won't be able to boot Windows anymore.
Try rebooting again, and see how it goes. It might not work due to the bug mentioned above.
You could also try replacing SYSLINUX 3.86 with SYSLINUX 4.02; it is a bit tedious and you'll have to do it all again when GParted updates itself unless they upgrade to SYSLINUX 4.x. Leave a comment here and I'll write up some further instructions for doing that if you're interested.