In TODO file I see
"""- oclHashcat-plus: integrate hccap converter for WPA2 into the GUI.
See if we can use the converter inside the GUI using an appropriate
http request instead of just opening the web page"""
Why is using online websites necessary? Bundling and using some files from Aircrack-ng distribution both for Windows and Linux along with the GUI will solve this problem.
Bundling them would cause additional maintenance work (different executables for OSes, additional dependencies when you build the GUI from source etc.) and should the format of hccap ever change (unlikely, I guess?) using the script on hashcat.net would take care of it for GUI users, too.
Also this essentially means forking aircrack-ng into the hashcat-gui github repository (unless we use one version and never update, just for the sake of hccap).
IMHO the "proper" (and rather easy way to implement) way to go here is to open a dialog in the GUI where you can select a file which is then uploaded to the hashcat.net converter and saves the output locally (without using the web browser of course, I am talking about making the HTTP calls and request handling in the GUI, can't get much easier for users then).
On the other hand I am currently rebuilding the GUI to have a configuration dialog so you can point the GUI to your local hashcat installations (we want to stop distributing the hashcats within the GUI archive for various reasons), I can imagine to put an option for a local aircrack-ng executable there, too, so people who have it locally can use it for converting.
Hope this answers your question.