Below are links to some of my favorite online maps for Guru Maps:

OpenTopoMap

Thunderforest Outdoor

Michelin

Bing Birds Eye

Google HD (without labels)

Google HD (with labels)

Source and many more maps: https://anygis.ru/Web/Html/Download_en?app=GuruIOS

Some more options:
https://ms.gurumaps.app
https://custom-map-source.appspot.com