Sim800 FCC Testing for GPRS
The "continuous transmission" requirement cannot be met with regular HTTP requests, you are expected to use every GSM slot, ignoring any higher-level protocols.

This test verifies that the emission spectra are within required parameters; this is done by moving a bandpass filter across the spectrum and writing down the power of the signal received. As moving this filter requires some time, the signal must be present for the entire duration -- interruptions could hide problems.

You are still required to follow the GSM framing and disable the amplifier between packets to verify that the sharp transitions at the packet boundaries do not translate to transmissions outside the allowed range.

The module is already approved as far as I know, so there is most likely a non-functional testing variant of the firmware that was used in that certification, so you'd have to ask the vendor for either a copy of that firmware, or a pre-flashed module that can be used for verification.