While, as Tom says, you can use GT0 15, you can use any good insulated wire. You can use the core from the coax if it isn't foam core. The wire should run a straight as possible. The best place for the autotuner is right under the deck near the backstay.

I have seen a few HF installations where the antenna tuner could not get a good match because of using coax as a feedline after the tuner. Replacing the coax with a single conductor solved the problem. If you are having problems getting a good match on certain bands this will probably solve it.

PS. The last time I needed GTO 15 I went to the Worst Marine in Anacortes. They had no clue what it was and had to look it up. It was special order only. They are as bad as Radio Shack these days.