When i was doing pixel art I very much enjoyed having full control and range of the tools I would be using, that said my stuff never looked as good when people would come in with photoshop and throw down some work and make it all pixelated after. There is a guy i use to follow "Rakilonn" linked you the profile, he is amazing at pixel art and very much worth reading about.

I would suggest using photoshop if you have it. The ability to manage your layers is a very good tool ( btw. gimp can do pretty much do anything photoshop can so if you have that you can use that as well ) and seriously unmatched when it comes to testing out different styles and looks. Anyway, great advice so far just practice with the tools you are using. Photoshop has the option to make hard edges... so you can certainly get that pixel look you are going for!