emil2424

The m.2 slots and the gpu won't affect each other, the m.2 slots all run off the chipset lanes.
 
You can run both m.2s at pcie x4 speeds, but there limited by the chipset link, so there is a total of 32gbit to both drives. But this is very unlikely to be a issue.

If you have no gpu you can run them full speed, or set the gpu to x8 (minimal if any performance impact) to run them at full speed.
Otherwise, chipset has 24 pcie lanes, with a gpu taking 16 lanes, you might think that means 8 lanes to split between your nvme drives, but alas other communications happen over those channels. So only one m.2 can run full speed with a gpu in x16 configuration, the other will be limited to x2.

For gaming, 3.0 x 16 will be plenty for a long, long time. Don't even worry about that at all. It will take MANY generations for this to be an issue.

Unless Ampere is a ridiculously impossible leap forward in technology, a 16x PCIe 3.0 slot will be more than enough.  The 2080 Ti was just barely over the 8 lane bandwidth usage.  The 3080 Ti will probably be somewhere in the range of 10-20% faster (just based on history), so it's not an issue.