The G key creates a secondary. You hold down the G and drag one clip next to another that is already a connected clip, but not a secondary storyline. You can drag it to the front or rear of the clip.
To just add a clip to a secondary, just drag it to the already existing secondary and place it where you like. You are correct, if you hold down the G key unnecessarily in this case, it will jump to the end. But there’s no need for the G. Just drag your clip to the front of the secondary. It won’t extend the secondary to the left though. It’ll ripple everything down. Which is pretty much never what I want.
If you want to add a clip to the front of the secondary, just snap it to the front, then highlight it and the secondary clips and press cmd+G and it’ll extend the secondary to include the clip in the front.