What do the abbreviations mean in Dragon Ball Z?
Yes, you are right. SSGSS stands for Super Saiyan God Super Saiyan. I know it doesn't make any sense, but according to the info I gathered, "Super Saiyan God Super Saiyan, more commonly known as Super Saiyan Blue, is a form that can be accessed after absorbing the power of Super Saiyan God and then combining it with the Super Saiyan form." 

For other forms like Super Saiyan 1 and Super Saiyan 2, no fancy abbreviations are used. They are generally known as SSJ/SS for Super Saiyan 1, SSJ2/SS2 for Super Saiyan 2, and so on.
There are also other forms like USSJ (Ultra Super Saiyajin), which are sometimes confused with Super Saiyan 2 or 3, but note that they are different. You can find more details on this site also.