The camera bag is not air tight. The silica gel will be used up in not time. No point. As long as you don't keep your camera stuffed into the bag for weeks without usage the chances for fungus are small. Use it every day, that's it.
its no use using silica gel in camera bag. it will turn pink within a day very quickly, coz your camera bag is not dry. However you can bring along a big zip lock bag, put the silica gel inside. When you not using your gear, put them into the zip lock bag, it will serve like airtight box and not bulky.