Compressor surge is when the engine cannot consume all the air the compressor is generating and the air tries to go back out and you get a fluttering noise, much the same as when you shut the throttle and have no dump valve.

Not what anyone has described this far.

And when your injectors are well maxed out and mixtures very rich at 1.3 bar, why are you even trying to run 1.5 bar ?

And why are you running the gain so high ? That's probably what is wrong. Leave the gain at zero until you have got your boost levels correct.
The gain on most boost controllers blocks the signal until a certain level, so running it too high will see boost oscillation which is what you have. Not surge.

You do not control boost via the gain ! Gain is there to try and improve spool up.

Tune the thing properly !