Ways to add proper ventilation to attic with ridge vent on roof?
I would add soffit vents. If it's wood soffit it's fairly easy to cut the holes with a small rotary saw or an oscillating multi tool. Then screw on prefab vents from any outlet supply. If it's aluminum soffit I've replaced solid panels with vented also and you could space them out and put in as only as many as desired. If it's aluminum fascia on front then removing the fascia first makes removing and reinstalling panels is a snap. Just first remove the aluminum fascia (on front) by pulling the bottom nails with pliers. I always like to screw the alum fascia back on with small stainless Phillips screws.

But if it's wood fascia that usually hangs down past the soffit in front, it becomes more problematic. I don't want to try and explain that to you other than give you a heads up that application can be more of a problem replacing alum soffit panels.

Oh and if the gable vents are not leaking in blowing rain, just leave them, they will just give more top level air escape. I see no need to seal up the gable vent unless you see evidence of gable vents taking in water in a blowing rain (some do, some don't) If that is not a previous problem then just leave them. The will have no negative effect on ventilation in any option scenario.