Instead of trying to push the Gmail app on your phone past its capabilities, try a web browser to log into your iCloud mail service within your Apple account. Look around for any option to allow deleting emails using third-party, non-Apple email clients.
Given our current state of privacy and security weaknesses I'd be surprised if there is such an option. Apple manages its integral services pretty tightly bolted down, so what you're attempting to do, remotely tweak an email server management setting using an Android app, is expecting a lot.
But it doesn't hurt to take a look, and if you don't look into your iClound Mail settings this is a good way to see if there are any options you can enable/adjust to better suit your needs.
Basically, what you're asking to do is actually an involved task. Email as a service has a long history and for decades now it's based on three protocols -- IMAP and POP for incoming messages, and SMTP for outgoing messages. All consumer email services (internationally) have wisely opted to use those three protocols as a universal standard. So exchanging email messages between different apps, services, and operating system platforms is still seamless and does not involve any user interactions.
Apple servers forwards email messages using SMTP, you receive them as IMAP, and vice-versa, your outgoing messages go through SMTP and are received by Apple's servers as IMAP. That exchange is the easy part that you see via your Gmail app.
Deleting them off the Apple servers crosses a line from simple email exchanging to email management.
With Apple, it curates its own environment closely, using monitored security procedures. To a degree not possible with Android. That's a big deal to some, a peace of mind matter to a lot of Apple users.
Facebook
Twitter
Instagram