This may not be new news but If you bought Spartan Assault & Spartan Strike for iOS, Android or P.C (Steam) you should be able to link it to your GT next time you login and get the achievements credited towards your Gamerscore.
I played Spartan Strike (then checked SA) on iOS and it now allowed me to sign with my MS login =}
https://www.halowaypoint.com/en-gb/games/halo-spartan-strike
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I have yet to play Spartan Strike on windows, I have it installed but the furthest I've ever gotten is to the main menu before I had to leave. Haven't went back to try it since...I will one of these days though.