One of the things that will make a difference is how old the child is when they come in to your life. If they were young, as my grandchild was at two years old, I can't see that there should be any difference. But if they are fifteen, it will probably be much more difficult for both you and the child. Children in their teens are not as quick to accept as a two year old is. Also there may be extenuating circumstances such as resentment of the biological parent's new family. This will not help to pave the way for a good grandparent/grandchild relationship.
Also, if they do not live with that biological parent, it makes it even more difficult for a grandparent to establish rapport with their new grandchild. Especially if the biological parent they do live with is negative about the child becoming too close to the new family, and particularly if it extends to a grandparent.
But whatever the relationship that the step-grandparent has been able to, or not been able to establish, it is absolutely necessary to treat all children equally. If a gift is bought for one, a gift must be given to all, and of equal value. It may be more difficult to be demonstrably affectionate with both biological grandchild and step-grandchild if the child is standoffish but some kind of an effort should be made. He may not want the affection initially but in time he may feel more willing to accept it. It will be important, however not to push yourself on a child until they are ready to accept you. In in any case overtures should be made even if it is just a pat on the hand, a squeeze on the shoulder and a smile. It will be important for them to feel that you consider them to be one of your grandchildren too. They should be included in any scheduled events or holidays instigated by the grandparent too. The decision may be theirs not to attend but the invitation should be sincerely given.
Another thing that will make a difference is the grandparent's relationship with the new spouse. The new spouse may discourage your involvement with their child or even, although hopefully not, your involvement in your biological grandchildrens' lives.
Being a grandparent can sometimes be a bit of a tightrope walk anyway, depending on the various relationships involved, but being a step-grandparent can be even more difficult with a new and older step-grandchild. The most important thing to remember when in this situation is to be fair, loving, accepting and interested in your new grandchild.
Having said that, love does take time to grow but it shouldn't take away from behaving in a loving way with the child. But being fair is the all-time important thing to remember. Children will pick up on unfairness immediately and it will be difficult to get on good footing with them if they have perceived your treatment has been unfair. Bottom line is, children are children no matter whose they are, and they all deserve to be treated equally and lovingly.