The time channel is already calibrated in ns. So for 5 GSPS, the time scale goes from zero to 200. Concerning your other issues I will come back to you later.
Another issue that we are having is with the data set stored on the .txt file looks incorrect. The time channel stops at 200 (but we think it should go up to 1024). In addition, the voltage channel appears to hover around small values near zero (as if triggering on noise). The output file appears this way even when we change the threshold to much higher values. It suggests that the trigger threshold is not actually being set? There are events where the output voltage appears to oscillate through huge negative and positive values too. So not sure what's going on.