First, I would gently correct the assumptions of your question. "Syncretism" implies the meeting and assimilation of multiple more or less fully formed systems. So we might be able to call certain strains of gnosticism "syncretism" between Judaism and/or Christianity and Neoplatonic philosophy, because Judaism and Neoplatonic philosophy are largely independent systems (begging Philo's forgiveness).

But the relationship between Christianity and Greek and Roman polytheism isn't syncretic, because Christianity wasn't an independent system that encountered GR polytheism at some later point. Some of the earliest roots of Christianity are in GR polytheism. Christianity didn't "combine" with polytheism, it grew out of it, just like it also grew out of Judaism. Most of the authors of our earliest Christian texts are or seem to be Jewish, but much of their audience seems not to be. The people that Paul's letters are addressed to, decades before even the first of the Gospels, are Roman or Romanized, not Jewish. We shouldn't make the mistake of thinking that just because we're not reading their direct writings, they weren't driving the development of the tradition in very real ways.

One example is the titles of Jesus. "Son of God" is a Greek title for Augustus. "My lord and my God" (Jn 20:28) seems to have been a specific address demanded by the emperor Domition. "Lord" (kyrios) is a pretty generic way to address rulers and gods, but that's important, too. Greek and Roman tradition had all sorts of generic ways of thinking about or behaving in relation to gods, and a lot of those show up in Christianity, because they were part of the base culture that most Jesus worshippers grew up in. Sometimes those overlap with Jewish modes, and sometimes not, although it's important to remember that Judaism was very, very inescapably influenced by Hellenistic culture by this period.

Another example, probably, is the form that "churches" took. John Kloppenborg has done a lot of work on what we call voluntary associations, which are sort of but not precisely like the Masons and the Elks in the modern United States. This was a really normal way for non-elite people with shared interests to come together and support each other. They could be trade guilds, neighborhood or family associations, cultic or ethnic associations, or a combination. It's likely that the earliest non-Jewish Jesus communities structured themselves in the same way, and some may have started out as associations that were already convening for other reasons.

As we get into later centuries we also see quite a bit of art that demonstrates the ways that Christianity was culturally embedded in the GR world, and that's probably going to be a lot of what you're looking for. There's a kind of "vocabulary" to the way that gods are depicted in art, and art featuring Jesus tends to follow it. Halos seem to develop out of or at least in reference to the gods Apollo and Helios, who are often shown with the rays of the sun around their heads. It was also common to portray people or gods using symbolism connected to another god, in order to communicate some shared function. Jesus as "the good shepherd" is portrayed exactly the same as Hermes Kriophoros. Some depictions of Jesus as a healer use symbolism from art depicting the Greek healing god Asclepius. Depictions of Mary as Jesus's mother are sometimes indistinguishable from depictions of Isis as Horus's mother. But again, I think it's less accurate to talk about syncretism than it is to understand Christianity as partially a development out of GR polytheism, and rather than "adopting" GR ways of portraying or talking about Jesus, it simply never discards them.