IF it worked before and now it does not, maybe it's busted. As you know, you have to hold the device (garage door opener) that you are cloning into the Homelink, very close to the Homelink. At this point I'd be thinking about whether the Homelink's antenna lead was broken or disconnected, and whether I could fix that or if I just had to get a junkyard replacement.
For the rest of you reading this: see this (blissfully short and succinct) video, for some GD openers you need a 'bridge' adapter: https://www.youtube.com/watch?v=pqi5lJPHl-E The video explains which brand and model year openers you need the bridge adapter for.
