The Gnostics: The First Christian Heretics is now available in Spanish. In fact, it was first published in Spanish in 2007, and I have only just found out. The reason I didn't find out was that Amazon kept listing the book as unavailable, and I assumed that it didn't exist. It was only when I found someone actually selling a copy on Amazon.com that I realised the book did indeed exist and was not just an error in Amazon's database.
You can buy a copy of the book from Amazon.com. (It's cheaper than Amazon UK.)
You can buy a copy of the book from Amazon.com. (It's cheaper than Amazon UK.)