Does Ravnica exist in the same universe as Toril? Is it possible to visit it with a Gate spell, for example? Or maybe it is in the very different universe?
There's no simple answer to that.
If both Ravnica and Toril exist in your campaign then your group has to decide what their relationship is.
As far as Ravnica's lore goes, it exists in the same multiverse as the various Magic planes (Dominaria, Zendikar, etc.) and has a long history of covert planeswalker visits. If you're lucky enough to be capable of planar travel and in the same multiverse, then you can come and go freely.*
However, the Magic planes are hard to mesh with D&D cosmology because there isn't "the" Material Plane, "the" Elemental Plane of Water, "the" Hells, and so on. The top-level divisions of the multiverse are the planes, which are all material and are different worlds. If your plane has gods then they're somewhere on the plane (like the Theros gods who live on an actual mountaintop, or the Kamigawa gods who are just everywhere).
Other metaphysically-connected regions function like demiplanes attached to the plane. For example, Innistrad has a Hell where demons come from, but it can't be a separate plane because you can't enter Innistrad from other planes, and the demons still get in just fine. Theros has an Underworld where you go when you die, but it's only connected to Theros.** These regions can connect to their "main" realm by temporary or permanent portals, whereas true interplanar portals are virtually nonexistent these days.
So to fit Toril or the other "standard" D&D settings into this structure you'd need to massage the concepts a bit. One option is for the whole World-Tree to be a single true plane, and its "planes" to be demiplanes like the Theros Underworld. This keeps its metaphysics from interfering with any of the other planes. There isn't a Feywild mirror-version of Ravnica, the demons of Innistrad aren't from the Abyss, and everyone from Tarkir doesn't get stapled to the Wall of the Faithless for being atheists. It also means that magic such as plane shift and gate that traverses the D&D "planes" can continue to work, without it stepping on the toes of "real" planeswalking.
At the other extreme: Toril is just one planet on the Material Plane. The other "planes" can be other planets, maybe orbiting other stars, reachable by the shortcut of astral travel instead of going the slow way through interstellar space. In this case their metaphysics probably do connect up, and this will have to be reconciled: Do the gods of Toril only care about Toril? Since the resurrection spell works in Toril, can people who die elsewhere be raised there? (Death in Magic is otherwise fairly permanent.)
Somewhere in the middle: You can treat all of the D&D "planes" as true planes, clustered together. Clusters of planes do have a few precedents in Magic cosmology: Alara, the Lorwyn/Shadowmoor duality, and (going really far back) the Shard of Nine Worlds; and in all of those cases it's considerably easier to move between the planes within the cluster than to anywhere else. From the perspective of someone on Toril, the Elemental Plane of Fire is an Inner Plane, and the Nine Hells are an Outer Plane, and Ravnica is an Even Further Out Plane, but all of them are separated by the Astral and a planeswalker should be able to reach any of them.
*Unlike certain planes that have barriers to entry (Innistrad) or exit (Ixalan, Shandalar).
**Notably, you don't have to be from Theros to end up in their afterlife, but you do have to die there.
Core Ravnica is in the Blind Eternities of M:tG, which is a very different universe. Guildmaster's Guide to Ravnica is there to support running games of 5th ed in that universe. Now, nothing stops a DM from saying "Ravnica is an alternate Prime Material Plane" or "Toril is in the Blind Eternities" and connecting the two together. It would be pretty easy, in either cosmology. Canonically, though, they're not the same. There are no (canon) planeswalkers on Toril.
The Blind Eternities do have some similarities with the Astral and/or Ethereal planes, but the Blind Eternities are way more hostile to life, and function in ways that are clearly distinct.
Is travel possible? Affirmative for Planeswalkers.
The Plane Shift: Amonkhet write-up is available as a free pdf from WotC. While not legal in Organized Play events, it is still "official" D&D-5e-compatible material. On the last two pages, it has an appendix titled "Planeswalkers and the Multiverse". It leaves it up to the DMs to decide on how to make the MtG multiverse (which includes Ravnica) and the classic D&D multiverse (which includes Forgotten Realms) "compatible", but it seems to be supportive of the idea. Quoting:
So can Planeswalker characters travel from Amonkhet to whatever plane the Forgotten Realms lies on? That’s up to you. The Plane Shift series more or less assumes a certain continuity from one Multiverse to the next, even as (for example) it makes no attempt to model Magic’s five colors of mana in the D&D magic system. So there’s no real reason an elf from Evereska couldn’t “spark out” and find herself on Kaladesh, as long as it works for your players and your campaign.
Can it be done with a Gate spell or similar? Hard, but ultimately up to you.
Planar travel in the MtG multiverse is currently an exclusive province of Planeswalkers. Another free pdf from WotC is Plane Shift: Kaladesh (PS:K). On page 11 we read:
Planar Travel. Items that allow travel to different planes of existence have no aether-powered equivalents. In the Magic Multiverse, travel from world to world is the exclusive province of Planeswalkers.
Yet in the past, MtG multiverse had its share of planar portals as well. (Thanks to @nick012000 for his reminder. There was even a card named Planar Portal.) Yet such portals stopped working after the Great Mending, except for the newly built Planar Bridge, which has limitations carrying organic matter.
Page 5 of PS:K describes aether as "a raw form of magical energy that fills the space between planes". How compatible aether is with the Weave and whether a Gate like spell would be able to work is up to you.