Loose gravel is one of those surfaces that looks stable until a bench gets a little sideways pressure. If you just set the legs on top, they will sink, slide, and start tearing up the gravel every time someone sits down. I have had the best results by doing two things: compact the base and anchor the bench to something below the loose layer.
Start with the gravel itself. Pull the bench away and dig out the top 4–6 inches where the legs will sit. Fill that area with angular crushed stone like ¾” minus or paver base, and compact it in 2-inch lifts using a hand tamper or plate compactor. Angular stone locks together much better than smooth pea gravel. This alone makes a big difference because it gives the bench a firm pad instead of a loose pile.
For a permanent installation, concrete footings are the strongest option. Dig holes through the gravel and into the native soil below, ideally below your frost line, or at least 18–24 inches deep. Place a cardboard tube form, fill it with concrete, and set galvanized post anchors or J-bolts before it cures. Once the concrete has set, bolt the bench legs to the anchors. This method resists frost heave and heavy use, but it does take more time and digging.
If you want a strong hold without pouring concrete, screw-in ground anchors are usually my pick. Heavy-duty helical or screw anchors can be driven through the gravel into the soil underneath. Use a breaker bar or rent an anchor driving tool to get them deep enough. Then connect the bench legs with galvanized brackets and bolts. This works well because the anchor grips the soil below the gravel, not just the loose rock.
For a lighter bench or a spot where you do not want to dig deep, large concrete pavers are a practical solution. Use pavers that are at least 24x24 inches, set them on the compacted base, and get them level in both directions. Drill into the pavers with masonry anchors or concrete screws, then fasten the bench legs to the pavers. This spreads the load over a wider area and keeps the legs from punching through.
If you are dealing with a temporary setup, or you cannot anchor into the ground at all, use ballast blocks or heavy steel base plates. Bolt the bench securely to precast concrete blocks or thick steel plates, then cover the bases with the surrounding gravel. The bench stays in place because the base is heavy and wide, but this method is not as rigid as a true ground anchor.
If I had to choose one approach for a typical street bench on loose gravel, I would go with a compacted gravel pad plus screw-in ground anchors. It is fast, strong, and avoids waiting for concrete. For high-traffic public areas, though, concrete footings are worth the extra effort. Use hot-dipped galvanized or stainless steel hardware, level everything before final tightening, and check it again after a week or two because the gravel will settle slightly.