I have reinstalled everything and switched the OS from nvme0n1 to nvme1n1 just to make sure whether there is any bottleneck because of the PCI topology. Unfortunately, it did not help.
I was running the following commands in 3 separate terminals simultaneously:
sudo ./gdsio -D /mnt/nvme0n1/gds_dir -d 0 -w 8 -s 1G -i 1M -x 0 -I 0 -T 300iostat -cxzk 1nvidia-smi dmon -i 0 -s putcm
The result of the both monitoring commands is attached as a PNG. The monitoring results show that %iowait of the CPU is relatively high. Is this already an indication that the data is redirected via CPU?
I also checked if the results from fio makes sense at all and the manufacturers specifications for the Samsung 980 (non-Pro, a mistake I did above) is a read/write throughput of ~ 3.5/3.0 GB/s. So I guess the results from fio are for some reason not suitable for comparisons to gdsio or my used CLI arguments are wrong.