The advantage of DTG is if you want to be more flexible, more print on demand, and not have money tied up in printed inventory that may never sell (this is assuming paying to have it printed ... owning your own is unfeasible unless you have enough volume to use it every day). Yes, it can very nicely reproduce gradients and a broad color range ... sometimes. It depends on the specific image and the specific DTG. You can tweak your art to partially makeup for the limitations of the printer, but some images will present much larger problems than others. I recently did one that featured an orange, and the DTG totally lost the darker dimples in the orange that gave it texture and made it look real. Turns out flavors of red are a weak point of the DTG used to print my DTG designs.
Screen printing would be cheaper per unit, if you produce them in quantity. You can reproduce art with screen printing, but it may take a lot of colors (so a larger press, more screens, films, and set-up and clean-up time). Separating the art and creating the halftones for such an image is not a trivial undertaking. Neither is printing it. Need to crawl before you fly ... to attempt this as an inexperienced screen printer would be to quickly snuff out your screen-printing ambitions.
If printing on WHITE garments is an option, then you might consider dye sublimation (polyester garments only), or JPSS inkjet transfers. If you need to be able to do dark garments, then neither of these is an option.