Hm, I did not realize there could be a problem writing two files. I will write it to just one file and compare the difference.
I did not post the entire loop code, the first line after void loop() is:
File GPSFile =SD.open("GPS.csv", FILE_WRITE);
So it opens every run of the loop.
Regarding the (gps.location.lng(), 8 ), serial printing that value gives longitude to 8 decimal places. However,
longitude = (gps.location.lng(), 8 );
then
Serial.print(longitude)
Prints 8.00 in the longitude column. Ideally, I'd like to write 0's for longitude, latitude, time, speed, and date in those columns when not getting a fix from the GPS receiver. However, if it just outputs the same data point over and over again for the duration the fix is lost, that wouldn't be the end of the world.