TL:DR = Yes
Your question is a good one, especially with the introduction of Private Relay. This feature is part of iCloud+ subscription and it provides an additional layer of security when you're browsing the web on Safari or using Apple's Mail app. However, even with this feature, the content of your Gmail messages wouldn't be protected by Private Relay. The reason is that Private Relay specifically encrypts your IP address and Safari browsing activity so that no one, including Apple and your internet service provider, can see or access that information.
To answer your second question, prior to using Private Relay, websites, advertisers, and potentially malicious parties could potentially see your IP address. This doesn't mean they could access the content of your emails or files on your iPhone directly, but they could track your online behavior and potentially target you with personalized ads. Private Relay essentially blinds them from seeing this information.
In terms of hacking, it's important to remember that no system is completely immune. Always maintain good security habits such as regularly updating your device, avoiding clicking on suspicious links, and using strong, unique passwords. Gmail itself also offers numerous security measures such as two-factor authentication which you can utilize to protect your account.
References:
1. About iCloud Private Relay - Apple Support
2. Make your account more secure - Google Account Help