Looking at your Gigabyte Motherboard Specs concerning PCIe x 16 slots and M.2 slots: https://www.gigabyte.com/Motherboard/B450M-DS3H-V2-rev-1x/sp#sp
Since you have a Ryzen APU it should be no problem installing a GPU Card in the correct PCIe x 16 slot, the one closest to the Processor.
As soon as you install the GPU card, BIOS will automatically set the GPU card as your Main Display Adapter unless BIOS is specifically set to IGPU and not to "AUTO".
So, No need to disable IGPU in BIOS. That way you can have the GPU as your Main Video Adapter and your IGPU as your Secondary Video Adapter. If anything should go wrong with your GPU card you can hook up your Monitor to your Motherboard to continue to have Video Output until you repair or replace your GPU card.
Normally BIOS is set to "Auto" so I would connect your monitor to the GPU card when you install it. Once you enter Windows go to Device Manager and see if both the IGPU and your GPU Card are enabled and without any errors.
Also make sure you have the correct PSU installed that would be powerful enough to run your GPU card without any Power issues. Otherwise you might need to upgrade your PSU depending on your GPU card.
As for the NVME Drive I would look at Gigabyte's QVL List SSD SUPPORT LIST and see if it is listed or not. That way you can also get a general idea of which drives are compatible with your motherboard: https://www.gigabyte.com/Motherboard/B450M-DS3H-V2-rev-1x/support#support-doc
Plus I would open a Gigabyte Support Ticket to make sure your NVME Drive is compatible with your Motherboard and GPU Card installed.
Your MoBo Manual should explain which type of drive it supports in which M.2 slots if not check with Gigabyte Support.
