GPP typically does not increase speed.
It increases work capacity.
I don’t advise speed work after a long layoff.
I do advise lots of GPP though.
The higher your work capacity, the more work you will tolerate as you ease back into training.
GPP should be somewhat low intensity and longer duration.
It is not simply cardio.
It can be a lot of things besides cardio.
Sled drags, circuits, hikes, etc are all forms of GPP.
Once you get back in shape and have been training, specific speed work such as:
Plyos
Dynamic barbell work
Jumps
Bounds
and more
All have their place.