What is the best alternative to GStreet - Street Map Viewer on iPhone?

Street View - street view maps, S tier, #1 in street view.

Is there a free alternative to GStreet - Street Map Viewer?

Yes: Street View - street view maps (S tier, #1) is free to download, with in-app purchases.

Yes. 3 apps rank above it in street view: Street View - street view maps (#1), Google Earth (#2), MAPAS: Live Street View & GPS (#3).

GStreet - Street Map Viewer is C tier, #4 of 7 in street view. Here is everything that competes with it on iPhone, in tier order.

1
Street View - street view mapsShigher tier
Free + IAP · ★ 4.6 · 9,822 ratings · score 90
Shows street panorama views and a live map for searched addresses, landmarks, places and your current location.
2
Google EarthAhigher tier
Free · ★ 4.0 · 47,490 ratings · score 88
Google Earth is a mapping app for viewing satellite imagery, 3D terrain and buildings, and Street View, and for…
3
MAPAS: Live Street View & GPSAhigher tier
Free + IAP · ★ 4.3 · 4,672 ratings · score 87
Free navigation app for browsing street-level imagery, live cameras, 3D and satellite maps, and basic GPS tools.
5
We Camera 03 | Street View AppD
Free + IAP · ★ 4.1 · 5,121 ratings · score 59
Street View and map viewer with current location navigation, pins, routes, four map types, sharing, search and traffic.
6
Live Satellite View & GPS MapsD
Free · ★ 4.3 · 2,800 ratings · score 57
Free Navigation app listing satellite imagery of any location on Earth, with GPS tracking, map layers and offline.
7
3D World Map VRF
Free + IAP · ★ 4.5 · 7,386 ratings · score 43
Street view and 2D/3D world map app with VR 360° panoramic tours and VR live video, per its App Store description.

What is the best alternative to GStreet - Street Map Viewer on iPhone?: Street View - street view maps, S tier, #1 in street view.
Is there a free alternative to GStreet - Street Map Viewer?: Yes: Street View - street view maps (S tier, #1) is free to download, with in-app purchases.
Does anything rank above GStreet - Street Map Viewer?: Yes. 3 apps rank above it in street view: Street View - street view maps (#1), Google Earth (#2), MAPAS: Live Street View & GPS (#3).