In gdu_util_get_connection_for_display (gdu-util.c), a comment speaks about Mbit/s (which is probably true):

speed = gdu_util_get_speed_for_display (connection_speed);
/* Translators: Connection with speed information.
* First %s is the connection name, like 'SATA' or 'USB'
* second %s is the speed, like '2 Mbit/s'
*/
result = g_strdup_printf (_("%s at %s"), name, speed);

But in gdu_util_get_speed_for_display, the MB/s string is used:
str = g_strdup_printf (_("%.1f MB/s"), displayed_speed);