If my OpenVPN profile uses redirect-gateway, does that guarantee that all of my network traffic will be routed through the VPN tunnel?

Yes, but with some important exceptions:

• Many Apple services such as Push Notifications and FaceTime are never routed through the VPN tunnel, as per Apple policy.
• During pause, resume, and reconnect states (for example when transitioning between WiFi and Cellular data), the VPN tunnel may temporarily disengage, allowing network traffic to bypass the tunnel and route directly to the internet. If you are running iOS 8 or higher, consider enabling the Seamless Tunnel Setting in the OpenVPN section of the Settings App. It will make a best-effort to keep the tunnel active during pause, resume, and reconnect states to prevent packet leakage to the internet.