There are no hard and fast rules for using a shaving brush ....no one will make you grow your hair back if you use it with a gel.
It's all about finding out what works best for you in your routine. Some guys swear by using the canned foams or gels and spreading it by hand, other use a brush. As for me I prefer the old fashion solid soaps, a good mug and a quality badger bristle brush...but that's me and I enjoy the process/routine as much as the result.
Experiment...find what works best for you- in terms of both your results and making what can be a boring daily routine a bit more pleasant.