Truly we do not have to speculate, but just read the license.
https://www.gnu.org/licenses/gpl-3.0.de.html
- Conveying Modified Source Versions.
- c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.
Similar in version 2, with slightly different wording but the principle is the same.
You use one line of GPL code, your entire code becomes GPL.
So unless you code is doing everything from scratch, not using a single WP or CP API or else function/code, you must license under GPL.