agm-location-marker.ts

import { GoogleMapsAPIWrapper } from '@agm/core';
import { Directive, Output, EventEmitter } from '@angular/core';

declare var google: any;

@Directive({
  selector: 'agm-location-marker'
})
export class AgmLocationMarker {
  marker: any;
  map: any;

  @Output('positionChanged') change: EventEmitter<any> = new EventEmitter();

  constructor (private gmapsApi: GoogleMapsAPIWrapper) {}
  
  ngOnInit(){
    this.gmapsApi.getNativeMap().then(map => {
      if (!navigator.geolocation) return;

      this.map = map;
      navigator.geolocation.watchPosition(this.onPositionUpdate.bind(this), function(err) {
        console.log('Unable to get location:', err)
      }, {
        enableHighAccuracy: true,
        maximumAge: 1000
      })
    });
  }

  onPositionUpdate(position) {
    let pos = {
      lat: position.coords.latitude,
      lng: position.coords.longitude
    }

    // Emit position change event
    this.change.emit(pos)

    if (this.marker) {
      // Update marker position
      this.marker.setPosition(pos)
      return;
    }

    // Marker needs to be created
    this.marker = new google.maps.Marker({
      map: this.map,
      position: pos
    })

    this.map.setCenter(this.marker.getPosition())
  }
}

template.html

<agm-map [latitude]="lat" [longitude]="lng" [zoom]="13">
  <agm-location-marker (positionChanged)="positionChanged($event)"></agm-location-marker>
</agm-map>

template.ts

positionChanged(pos) {
  // pos = { lat: 1, lng: 1 };
}

Great directive.
I'd recommend to release resources on OnDestroy since watchPosition might execute even when this directive is destroyed.

   watchId: number;
   ngOnInit(): void {
        ....
        // Save watchId
        this.watchId = navigator.geolocation.watchPosition(this.onPositionUpdate.bind(this), (err: PositionError) ...
       ....
  }
  
  ngOnDestroy(): void {
    // Release resources
    if (navigator.geolocation && this.watchId) {
      navigator.geolocation.clearWatch(this.watchId);
    }
  }