It's the regular heptadecagon, i.e. 17 sides. The heptagon, or septagon, is inconstructible, with 7 sides, as is the nonagon (9).
Also Gauss didn't actually devise a construction. He just proved it was possible as the length a side in terms of the radius of circumscribing circle is "solvable in radicals". Actually, it's very interesting, and I've been studying it. You can quickly find animated versions of actual constructions on the internet.
Thanks!
I just checked my “Number Theory and Its History” by Oystein Ore (Dover Books, what else?) and see that it’s:
A regular polygon with n sides can be constructed by compass and straight-edge if and only if n = (power of 2) times distinct Fermat primes.
The first Fermat primes are 3, 5, 17, 257.
IIRC Gauss’s proof used complex roots of unity.