If I use just cut up chicken tenderloin does it need to be cooked first before adding in the water, onion and ginger?

Add water, onion, ginger along with our Chicken Flavored PHO Soup Base first and bring to boil. Then add your chicken tenderloin. If your tenderloin strips are raw make sure it is fully cooked in the boiling broth prior to serving. Thank you for your interest in Quoc Viet Foods.

Is this product Gluten Free?

Thank you for your interest, our Chicken Flavored Pho Soup Base does not contain gluten. Sorry for the delayed response.

How much of this should I use if I only want to make a bowl of Pho?

Thank you for your interest in Quoc Viet Foods. This jar is intended for make two pots of 2 gallons of Chicken Flavored Pho broth (4 gallons total with all the contents in the jar). Two spice bags are included per jar, the spice bags is what helps brings out the aroma and flavor Pho. If you only want to make the chicken broth alone without using the spice bags, you can make a 16 oz bowl of broth using 2 tsp (9 g) of Chicken Flavored Pho Soup Base.

Would this be good as a base for ramen if I add tonkatsu soup base as well?

absolutely - I do that all the time. It is also good added to Shoyu and Shio ramen soup bases as well if you desire a pork taste to those ramen soups.