Learn German: Wir finden überall Schutt und rostiges Metall entlang des Gitters. - We find rubble and rusty metal everywhere along the grate.

You are right that entlang is traditionally placed after the noun and takes the accusative (like das Gitter entlang). However, when placed before the noun, it usually takes the genitive case, as seen here with des Gitters, or sometimes the dative. This pre-positioned form is very common in written or slightly more formal German.

Why does the adjective in rostiges Metall end in -es?

Metall is a neuter noun (das Metall) and it is in the accusative case here because it is the direct object of the verb finden. Since there is no article before it to show the case and gender, the adjective rostig has to take the strong declension ending, which is -es for neuter accusative.

Is Schutt always used without an article?

Mostly, yes. Schutt (rubble or debris) is an uncountable mass noun in German, just like in English. You generally do not use an article with it unless you are referring to a very specific, previously mentioned pile of rubble.

What kind of structure does Gitter refer to here?

Gitter translates to grate, grid, lattice, or bars. It refers specifically to a crisscrossing framework, usually made of metal. In this context, it could mean a wire mesh fence, a heavy metal grate on the ground, or iron bars.