Firstly, let’s understand What is GPL?
GPL stands for GNU General Public License and denotes a family of licenses that are united by the idea of open source. It is one of the most popular forms of licensing for all kinds of software, most notably – the Linux operating system.
- GPL means that any work derived from the initial product can be freely distributed, but only under the same license terms (i.e. GPL).
What does this mean?
This means that the end-user is free to redistribute/resell the theme/software but under a GPL only. Or in simpler terms, a user may use a single theme on multiple websites.
Now, as far as the safety of the GPL themes is concerned you don’t need to worry about it.
The safety of a particular theme depends on how its coded/whether the theme authors have followed good coding practices or not. It has nothing to do with the license.
A non-GPL theme might also be unsafe due to bad coding.
Unsecure themes: So which themes are unsafe?
Pirated themes. Nulled versions of the premium themes available on shady websites. These types of pirated/nulled versions almost always contain malware that will make your site hackable in the future.
Do not ever use pirated themes/nulled versions of premium themes. These are the only unsafe themes that you need to worry about. And also themes of unverified publishers or unpopular publishers.