The achievement challenges on xbox 1 should increase your gamerscore.
Microsoft designed the system to include zero point challenges and gamerscore based achievements.
Xbox timed challenges seem to always be zero GP, but other titles may include a little bit bigger reward than a badge and title. But each of the challenges involves racing for which we get winnings anyway. I won’t stop doing the big races, but it would be nice if they at least paid out at the basic per-lap rate. Indy 500, for example, typically pays out 1000xp per lap; the Indy 500 was 375 to 500 miles, or 150-200 laps - would be nice if it paid out based on 150,000 - 200,000 xp instead of capping at 60,000.
That the same even when doing 2 hour races? A friend and I normally do 20 lap races to get extra coin, would doing a timed session not grant more then 60k coin?