The g_io_channel_new_file() and g_io_channel_unix_new() functions
set is_seekable flag for all character device files

glib2.0-2.30.0 % grep -nC1 -eis_seekable glib/giounix.c 
554-
555:  channel->is_seekable = S_ISREG (buffer.st_mode) || S_ISCHR (buffer.st_mode)
556-                         || S_ISBLK (buffer.st_mode);
--
632-  if (fstat (unix_channel->fd, &buffer) == 0)
633:    channel->is_seekable = S_ISREG (buffer.st_mode) || S_ISCHR (buffer.st_mode)
634-                           || S_ISBLK (buffer.st_mode);
635-  else /* Assume not seekable */
636:    channel->is_seekable = FALSE;
637-

And g_io_channel_get_flags() returns G_IO_FLAG_IS_SEEKABLE based on that

glib2.0-2.30.0 % grep -nC1 -e[^@]G_IO_FLAG_IS_SEEKABLE glib/giochannel.c 
1012-  if (channel->is_seekable)
1013:    flags |= G_IO_FLAG_IS_SEEKABLE;
1014-  if (channel->is_readable)

Now for example /dev/ttyX and /dev/input/eventX channels have the
G_IO_FLAG_IS_SEEKABLE flag, but calls to g_io_channel_seek_position()
will fail.