Wow. I'm shocked you still had the old version running that long. I'm pretty sure the 3.9.x versions stopped working for me with MC17 (or earlier). I remember at one point my license had lapsed and I had it stop working, anyway.
I'm on 5.0b on the PC I happen to be in front of right now, and it works fine with MC 18.0.190.
Did you try this, just to be sure?
1. MAKE SURE MC IS CLOSED (including the Library Server in the tray, if it is running).
2. Uninstall G-Force and Softskies (both).
3. REBOOT. DON'T SKIP THIS BECAUSE YOU ARE LAZY.
4. If MC auto-loads when it boots, make sure it is closed again.
5. Install G-Force and/or Softskies.
This might, maybe, if you're lucky, work.
Alternatively, you might be able to get it working via some elbow grease. If you install an old version of MC (even just in trial mode) and then install G-Force into that copy, you can "hack" it over to the new version by copying over the relevant dll files (and re-creating the proper Registry entries). There are instructions for really old versions of MC buried somewhere on Interact. I remember, because I wrote them at a time when a new beta of MC had come out and Andy hadn't updated G-Force to work with it yet.
Even better alternatively, just give Andy the $20. He deserves it for all of his hard work over the years, and it isn't much money. Plus, the new on-screen UI is much nicer for G-Force (it has the on-screen style like Whitecap does now). Also (while you're at it), his new visualizer Aeon is awesome. Give it a try.