A jump gate exists to get characters to the plot fast. They are constructed of unobtainium (possibly alloyed with handwavium). If you don't want to get meta then jump gates move vehicles or personnel much faster than normal. They let s avoid logistics that get in the way of the story.
Seen in this light the most famous jump gate is the transporter in Star Trek. Gene Roddenberry conceived the transporter to avoid having to show his starship or shuttles landing every week and the resulting cost (stock footage and mis-en-scene scripting not being as practical as matter energy conversion and beaming in the 60's).
the transporter was effective at moving the plot along though allowing senior vital officers to out themselves in danger almost immediately. The problem was it often allowed them to be whisked out of danger just as fast. So writers had to keep coming up with reasons the transporter didn't work. Either that or Kirk got clunked on the head and his communicator taken away. It made me wonder why they didn't just sew a tracking device into his uniform when I got older.
Larger jump gates work much the same way allowing ships to be teleported or warped much further than they could travel on their own or at least faster. Many settings have either ftl capable ships or jump gates but it doesn't have to be this way. The Mass Effect setting has a network of star gates connecting different clusters of stars across the galaxy with shorter ranged gates within each cluster and ftl ships too. The old Starfire game had interstellar travel made possible by natural space warps ships being strictly stl.
Jump gates are neat but as with the transporter you have to establish the rules for their operation beforehand and very carefully.
The first major objection to a jump gate only setting is that it makes it far harder for naughty pcs to get around. A jump gate is of enormous economic and strategic value. It tends to be guarded by large numbers of navy ships and unless a enemy fleet has come blundering along these bored officers have nothing better to do than board your ship, ask plenty of questions, go over all your licenses and stick their noses into every nook and cranny.
So piracy and smuggling will tend to be interplanetary unless the pirates and smugglers are very clever and have really good forging skills. Even then your contraband cargo might be noticed (micro size labels, molecular copyright codes, it's like no one has any trust anymore!)
This doesn't have to be the case. A space warp could be arbitrarily large to allow it's use. Maybe your ship mounts a drive to attract said warp or merely needs proximity for your speed boost. In that case patrol ships will have to be more active and your nefarious crew might slip away. This is right out if you have to be physically docked or flying through the machine/wormhole.
Of course some races might build their jump gates to resemble mouths or other things that will test the resolve of the first pilot to fly through. ("I ain't flying through anything shaped like a skull. Arrest me now.")
Most likely they'll be big old toruses because you have a tool function on most rendering programs to create toruses.
The reliability of gates boils down to two things 1) will it get me there and 2) will it be the 'there' I want? The gate not working means you check your math and circuit diagrams and try again. The accuracy can be a worse problem, You might get sent to another star system or reach your destination's cometary cloud. How good is your stl drive anyway? How good is your ftl drive for that matter?
If we find gates built by non-humans reliability becomes more complicated as you have to back engineer alien control systems and build a human user interface. Or the controls might have to be operated as is with little knowledge of where you' re going. The Heechee series by Frederik Pohl deals with similar problems though the humans are using ftl ships they discovered
Another factor is whether your gates require a receiver. A gate could be linked to another gate, or be able to project you anywhere. Perhaps linking to another gate is not necessary but increases range, accuracy efficiency etc. A one way jump gate is of little use unless you plan on staying in your destination or you have the materials to construct another gate. The old Justifiers rpg dealt with this theme.
Gates are a great way to increase the scope of a campaign. Suddenly you go from interplanetary to interstellar with all that implies (aliens!) In an existing interstellar campaign they can be used to present new threats or allies. It can also end with your players finding out what happens when you trigger one jump gate inside another.