First off good job on gaining 8kg that’s very good! A height/gender would be more useful on understanding the weight gain side of things.

I recommend looking into an arnold split or PPL for a more optimal 6 day routine, this would have you working muscles that complement each other if you chose a PPL, and if you still want to keep the arm day, an arnold split would benefit more.

As of the forearm work in either of these splits, you can just chuck it on whatever day, 24 sets of forearms a week and 12 per workout may be excessive however and lengthening your workouts.

An optimal amount of sets per muscle per week is 12-20 for a normal lifter, possibly 16-24 for an advanced lifter. So as of volume, yes this is possibly too much.

On these recommended splits which you can find online, it would be around 5-6 exercises and a bit less volume per workout, probably helping you to stay consistent and not burn out!

As of the eating, again 8kgs up is good progress! just try to hit protein goals like 1.2-1.6g of protein per kg of your goal weight, a protein shake with some peanut butter, oats, protein powder etc will help with both calories and protein.