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

Custom style

Stefano made a great custom style, similar to old school Michelin road maps. You can find it at https://heimathafen.one/guru-maps-custom-mapstyle
I’ve made some small tweaks to it:
https://egregius.be/files/gurumaps/Egregius v1.01.style.zip
https://egregius.be/files/gurumaps/Egregius v1.02.style.zip
https://egregius.be/files/gurumaps/Egregius v1.03.style.zip
https://egregius.be/files/gurumaps/Egregius v1.04.style.zip
https://egregius.be/files/gurumaps/Egregius v1.05.style.zip
https://egregius.be/files/gurumaps/Egregius v1.06.style.zip