The PSR-GX76 has MIDI IN/OUT ports but no USB TO HOST port, so obviously you're using a USB-MIDI interface of some kind. Can you tell us what it is?
I think most USB-MIDI interfaces these days are designed to be class-compliant so they won't need a driver, but if they do need a driver then it should be the driver that was written for that device, not the driver for the brand of keyboard. For example, if you're using Yamaha's USB-MIDI interface-- the UX16-- then you can use a Yamaha USB-MIDI driver with it (whichever one is designed for your operating system). But if you're using a non-Yamaha USB-MIDI interface then you won't be able to use a Yamaha USB-MIDI driver because the driver won't detect a Yamaha USB-MIDI device for it to talk to.
Note that many of the inexpensive off-brand USB-MIDI interface cables can be unreliable and problematic for various reasons, whereas well-known brands tend to be more reliable and less problematic. One of the least expensive USB-MIDI interface cables that a number of users have verified to work well with older models of Yamaha keyboards is the iConnectivity mio, so if you're using a cheap off-brand cable then you might consider replacing it with a mio.
Of course, it might be that the cable you're using is fine but there's another issue. For instance, you need to be sure that you've got the MIDI connectors attached correctly. Although it might seem counterintuitive, like does not connect to like when it comes to MIDI-- that is, you should connect the MIDI IN plug to the MIDI OUT port, and connect the MIDI OUT plug to the MIDI IN port (IN to OUT and OUT to IN, not IN to IN and OUT to OUT).
I also seem to remember reading something a while back regarding an issue with MIDI-OX not communicating properly with certain USB-MIDI interfaces-- something about the SysEX messages, if I remember correctly-- but I'd have to search for those old posts to refresh my memory. So if you've got the USB-MIDI interface connected correctly but MIDI-OX still seems to have problems, try using a different MIDI program to see if it works.