WowAce

What version of Grid2  are you using (Version is displayed in General/About Tab) ? V2.8.31

What game client version, classic or retail, windows or mac and language are you using ? Classic Cata

  
Did you try having Grid2 as the only enabled addon and everything else disabled? YES

Was it working in a previous version? If yes, which was the last good one? Incoming heals

Please provide any additional information below.

Grid is showing incoming heal as if a cast a full bar heal instead of 5 or 10% of the bar 
It fill the bar instead showing exactly the amount of heal i put on my mates

The game is bugged and  reports wrong incoming heals.

You can go to incoming-heals status configuration and reduce the heal amount.

There is a multiplier slider, so if you set 0.1 the heals are divided by 10.

But this does not correctly fix the issue because each heal spell is wrong by a different amount, some are x5 higher others x50.

Report the issue with the game in official  wow forums, only if enough people reports the issue maybe blizzard will care.