Did you use IMAP to begin with?
Only IMAP (and not POP3) provides access to server folders -- and for Gmail accounts, Aqua should have been able to identify the server spam folder, and should be showing "move to spam" in message list / message view.
The default for Gmail accounts in Aqua is IMAP, but to check:
Long press the account -> options and folders. If you see "IMAP options" (and not "POP3 options") then you're good.
And while you're on that screen (the account's "options and folders"), in just a few clicks you can make the Spam folder visible in the account list / sidebar. Just click "Folders", find "[Gmail]/Spam", tap it, and change to "Sync as Spam".