If Rageblade gives you 25 ap and 25 ad how will the Gathering Storm mastery know wich adaptive stats

Your other items.

The numbers are exactly even, so it gives you what your default adaptive stat would have been; ex: Leblanc will get AP, but Talon will get AD, if both were using Gathering Storm.

Based on your other items.

It was specifically changed to stop forcing Adaptive damage to slide over into AP every time.

All champions have base AD, no champions have base AP. So, I imagine the game will do something like this:

{
if totalAD > totalAP { totalAD == totalAD + modAD; } else if totalAP > totalAD { totalAP == totalAP + modAP; } else explode; }

What about on Kog'maw or Kennen

Shouldn't it select AD? I mean AD in general is more valuable than AP in a 1:1 ratio. I feel there should be a coefficient because obviously an AD not named Jhin has around 200-250 bonus AD lategame maximum, while AP's can reach values as high as 700+ AP, 1000+ if they're named Veigar

25 AD is more gold value than 25 AP (almost double) so the weighting of the system should adapt to AD.

Guinsoo's Rageblade COST: 3000g [AMP Tome + Recurve Bow + Pickaxe + 690G] +25 AD +25 AP +25% Attack Speed Passive: Basic attacks deal an additional 25 magic damage on hit. UNIQUE Passive: Basic attacks grant +2.5% Total Attack Speed, +2.5% Attack Damage, and +2.5% Ability Power for 2.5 seconds. Upon reaching 6 stacks, Rageblade is fully unlocked, granting Guinsoo's Rage. Guinsoo's Rage: Every other basic attack will trigger on hit effects an additional time. While at half stacks, melee champions' next attack will fully stack Rageblade

I thing this is best version of Rageblade.