Hi wanting to connect Nest and want to install a C wire. HVAC only appears to have G,W,R so not sure where to connect C.
Step 1: Find the low voltage side of the transformer. Using a volt meter in AC mode, measure the voltage between the two terminals circled in red, then measure between the two circled in blue. One of these pairs should show ~240 V mains voltage; the other pair should show ~24 V low voltage.
Step 2: Measure voltage from the R terminal (circled in orange) to each of the 24 V transformer terminals identified in step 1. You'll measure ~0 V to one of them and ~24 V to the other one. This latter one is your C terminal. Splice as needed to get a wire out from there to the thermostat.
By definition the R terminal is one end of the transformer winding and the C terminal is the other end. Simple thermostats (bimetal strip, or battery-powered programmable) simply short R to W or G to call for heat or cooling. Controls makers didn't spend money and circuit board space on the unnecessary C terminal.
Now fancier thermostats, which require so much power as to make batteries infeasible do require the C terminal. Newer HVAC controls include the C terminal, but when working with older controls, the only option is to find and tap into the C circuit somewhere.
The thermostat's processor and display are powered by current flowing between R and C. Connecting the C wire and the R wire both to the R terminal leaves you with 0 volts between the wires; the thermostat can't be powered from 0 volts.