Can I get banned on SG for arguing/insulting with another SG users OUTSIDE SG?

Why argue at all? Time's too precious to be wasted on such trivial matters. Block and forget, if you can't forgive. Out of sight, out of mind.

Actions taken in connection to elements directly related to Steamgifts (ie, attacking a user who is trying to give you a gift through Steam, or acting inappropriately within the SG Steam group) have, if I recall correctly, occasionally led to in-SG consequences. Most notably, the chat for the SG Steam group is explicitly stated to be covered by community rules within the site's guidelines (and that section of the rules seems to extend to covering the Steam group as a whole, as well).
Likewise, it's written in the SG guidelines that it's inappropriate to contact SG staff outside of SG for SG related matters, so that would also potentially run the risk of counting as a rule infraction. Moreover, Bans [versus Suspensions] have pretty much exclusively come about when someone attacked or harassed the site staff to excessive degrees after already being under Suspension, so that kind of trumps any other consideration.

However, more generally, SG policies seem to fairly firmly exclude off-site interactions for any sort of consideration. As such, while matters related to SG may or may not carry some small degree of risk, if it's something that only "could" be related, rather than something that is self-evidently related, then you should be safe (and for anything firmly non-SG-related, regardless of its nature, precedence indicates that you should most certainly be).

Though, frankly, the circumstance you're bringing up strikes me as being akin to very slowly punching an ant hill apart with your bare hands. There's no constructive purpose to the action, and everyone involved is sure to take unnecessary stress from the matter- and, if you push it too far, you're encouraging lingering self-injury regardless of how much you've been assured you wouldn't be at risk of such.

Calling out wouldn't apply in a circumstance, regardless of location, unless you were revealing information about the user that wasn't already clearly evident within the information already available in that thread or chat room at that time, or otherwise both publicly visible and readily apparent. There are plenty of other guidelines that poor behavior can end up being in violation of, however, without touching upon considerations of calling someone out.

Short answer; no.
Long answer; nooooooo.

only want to underline (no sarcasm!) that you're asking this INSIDE SG, and pretty easy a Mod will jump in... :P

argue with a steamgifts moderator is enough reason for suspension as "other sg offence". if you take a look on the comments you'll better digest the "argue" factor. in the other hand, the calling out mainly happens when you you expose anything pointing out the person. I think it don't apply, unless if you bring that to these forums.