John Gill Commentary Psalms 86:5

"For thou, Lord, art good, and ready to forgive, And abundant in lovingkindness unto all them that call upon thee." — Psalms 86:5 (ASV)

For you, Lord, are good
Essentially and independently good, from whom every good and perfect gift comes; good in himself, and good to others; good to all, in a providential way; and good to his own special people in a way of grace: this is asserted by Christ, (Matthew 19:17)

and ready to forgive ;
there is forgiveness with him, and it is to be had without difficulty; he has largely provided for it; he is forward unto it, he freely giving it; it is according to the riches of his grace; he does abundantly pardon; no sooner is it asked but it is had; this David knew by experience, (Psalms 32:5) ,

and plenteous in mercy unto all them that call upon you ;
in truth, in sincerity, in a right way, through Christ, and faith in him; to such not only the Lord shows himself merciful, but is rich and abundant in mercy; he has a multitude of tender mercies, and abounds in his grace and goodness, and in the donation of it to his people; all which encourage their faith and hope in their petitions to him.