On Snapchat, or any instant messaging platform, "WSG" simply means "What's good?" It can be used as a greeting when sliding into someone's direct messages or just when replying to a "hi" text message.
A "WSG" text doesn't necessarily have a specific connotation behind it and it's sort of situational. The same way that "LOL" can be interpreted as genuinely laughing, theoretically laughing, or sarcastically laughing.
Also, for the record, most people in person aren't going to be capitalizing their acronyms anymore. Don't let a Gen Zer catch you in the act or you might not be able to beat the cheugy allegations.
Most of the time "WSG" will be as friendly as casually asking someone "What's up?" either to check in on them or see if they have any plans. Typically, it will just be to see why you hit them up in the first place.
Of course, it can certainly take a more confrontational tone. The same way that "What's good?" can be used to ask someone if they have a problem with you, the acronym can certainly do the trick, too, though you're probably not as threatening through the screen.
More than likely, if you receive a "WSG" text, don't sweat it. Simply reply with what you're up to... that is if you want the conversation to continue.