The genecyst GYM log format is:

while (!EOF)
  switch (readbyte()) // readbyte() reads the next byte from the file
  {
    case 0: 1/60th of a second has elapsed; 
    case 1: ymport=0; ymreg=readbyte(); ymdata=readbyte();
    case 2: ymport=1; ymreg=readbyte(); ymdata=readbyte();
    case 3: write readbyte() to psg port
  }

Generator does not put sample data into GYM logs because it's impossible
for a player to know when to play the samples with only the accuracy of
1/60th second.  See GNM logging.