Marlin - slowing down print speed
G-code commands are not in the firmware. You're trying to find something in the code that simply isn't there. Your slicer is responsible for generating a g-code file, which you can then send to your printer (either by printing from an SD-card) or using a host program like repetier or cura. The commands in the G-code file are then executed by the firmware, but they're not part of it.

How the commands are actually processed is not trivial, and it happens in multiple places.

The commands are processed in the process_commands of Marlin_main.cpp. Upon reading a G1 command, it calls get_coordinates to parse the X/Y/Z/feedrate values from the command. Next, the prepare_move function is called, which applies the necessary transformations (clamping to endstops, and the reverse kinematics if you have a delta) before calling plan_buffer_line (which is in planner.cpp) which does some more preprocessing and computes a "block" which is pushed onto the buffer. Next, planner_recalculate is called, which iterates over the blocks to find feasible acceleration patterns for them. Finally, an interrupt (in stepper.cpp, ISR(TIMER1_COMPA_vect)) periodically reads blocks from the buffer and actually sends the signals to the steppers.