Is there a name, term, or idiom for this gesture, the one buddy christ is doing here? Or a name similar to this gesture?
Point and wink comes pretty close (here), but doesn't get you the thumbs up. In fact, the image you posted shows up in that search result.
A related gesture is finger guns, which is usually playful and positive and often accompanied by a wink (here).
In fact, if you google image search "Jesus finger guns", you'll get a lot of results showing the image of Buddy Christ.
I don't believe there's a single term for this. Rather, it's two gestures combined:
• Pointing and winking, indicating a personal or covert understanding
• "Thumbs up", indicating approval