Learn Italian: Anche digiunando spesso, Marco ingrassa comunque. - Even fasting often, Marco gains weight anyway.

This is an "implicit concessive clause." Because the subject who is fasting and the subject gaining weight are the same person (Marco), we can shorten "Even though he fasts" to anche + the present gerund (digiunando). This is a very common, advanced alternative to using anche se with a regular verb.

Could I use this same anche + gerund structure if the subjects were different, like "Even if I fast, Marco gains weight"?

No. You can only use the implicit anche + gerund structure when the subject of both actions is exactly the same. If you are the one fasting but Marco is the one gaining weight, you must use an explicit clause: Anche se io digiuno, Marco ingrassa comunque.

How would you rewrite this exact sentence using a regular verb instead of the gerund?

You would use anche se (even if / even though) followed by the present indicative: Anche se digiuna spesso, Marco ingrassa comunque. Because his fasting is a stated fact rather than a hypothetical scenario, you use the indicative mood, not the subjunctive.

Since anche already sets up the contrast ("even"), isn't adding comunque ("anyway") redundant?