Understanding S2S and GMP Integrations in Programmatic Ads

Can someone explain the differences between S2S (Server-to-Server) and GMP (Google Marketing Platform) Integrations? Additionally, if there is any reading material available on both subjects, please share. With the upcoming deprecation of S2S integration on AppsFlyer, what alternatives exist for apps that are not listed on the Play Store, such as Real Money Gaming (RMG) apps? These apps cannot utilize GMP for tracking data, so how will they manage to track their data?