If you download the weapons welder programme that is available, you can then download previous FPSC entity packs. Not all of them are free to use, but a lot of them are. Lots of WWII resources in a lot of them too.
The Weapon Welder programme allows you to convert stuff from FPSC to Game Guru ready, and can be found here:
https://forum.game-guru.com/thread/218121
And you can download all the free FPSC entity packs here. The downloads that say FPSC model pack and a number:
https://github.com/LeeBamberTGC/FPS-Creator-Classic
Here is a generalised list of what is in the FPSC model packs, just find the ones that say it has WW2 weapons and match it to the page of model pack DL links. Keep in mind, not every pack is available though:
http://fpscreator.wikia.com/wiki/List_of_Model_Packs
Weapons are very time consuming to make, so it's doubtful anyone will have a lot to give away for free, so dl'ing and converting FPSC weapons is your best bet.
There is a free WWII gun on the store though, so might be a good start for you:
https://www.tgcstore.net/product/28011
Hope this helps
I'm not a box of avocados, Logan