With two machines connected via serial cable, trying to send files between the two using gkermit both in 1. minicom to minicom and 2. minicom to mgetty, you must issue the gkermit recieve before the gkermit send. If you issue the gkermit send first, you will see some garbage on the other machines screen, and the transfer will fail.
I think this is normal. the receiving end can not signal the sending part when it is ready to receive data.