Is the attention where has you 1-1 with no distractions, completely able to absorb in and respond to him?
If not, smaller amounts but more of those sort of types of interaction might be what is needed.
If you are already doing that, can he go to a relative or nursery some of the time, so you can spend more time 1-1 with baby? As at 3yo he is unlikely to go off and play on his own for very long, especially if he knows you are giving the baby attention.
Gentle parenting is HARD, as much as the benefits are clear, but don't pressure yourself to always be 'that sort of parent' I have done fine gentle parenting both my girls til around age 2.5-3, however with DD1 who is now 6, I hold my hands up and say alot of that has gone out the window, especially with various life stressors of the past few years...unfortunately, whilst we are definitely still loving and responsive parents alot of the time, we do not manage to parent her that way quite abit of the time now, as she has got older it has got harder, because there is more refusal to listen/defiance when following instructions etc, so i do find we criticise, nag and raise our voices more than we should now.
The main GP things we always live by still though are that we always apologise and fix things after getting angry with her, we try to respect her own preferences, eg: with food or clothes, and we always encourage an open line of communication about feelings.