There's pizza dough recipe which calls for vital wheat gluten and I can't find it anywhere in India. Is there any substitute for it? Can I use seitan as a substitute?
No, there is no substitute for gluten, at all.
The gluten + soft flour combination is itself a substitute for bread flour, so if you can get bread flour, as Catija suggested, use it. If you can't, you need another recipe. Especially if your goal is to "not make it complicated", don't use substitutes. Substitutes are always complicated. The easy thing is to follow a recipe without making changes, and there are thousands of good recipes, so if you can't make one, choosing a different one is easier than looking for substitutes.
Seitan is not a substitute for gluten, even if you grind it up. In seitan, the gluten is already set, but you want it "raw" for making dough.
General purpose flour contains about the lowest amount of protein where you bring out the gluten through the process of keading. I start with a small amount of flour mix and add water slowly until it flows like pancake batter. Whip this quickly for several minutes to bring out the gluten and form long stretchy bands within the mix, then add small quantities at a time the remaining flour mix. Resting the dough also helps form the gluten.
Semolina flour contains a good quantity of protein, is good tasting, and firm. A combination of semolina and general purpose flour will provide the protein/gluten needed.