What does Grayson want to ''say'' to Maniac by giving the boy his old glove in Maniac Magee?

Grayson is telling Maniac that he loves him by giving him his old baseball glove.

Grayson is the old groundskeeper at the zoo.  He finds Maniac sleeping in the buffalo pen and takes him in.  Maniac and Grayson are perfect for each other.  Both of them have been alone, and they have each become the other’s family.  Maniac is sentimental, and understands that Grayson loves him like a son.  He appreciates what the baseball glove meant to Grayson, and he understands that by giving it to him, Grayson is telling him that he loves him.

To anyone else, it was a ratty old scrap of leather, barely recognizable as a baseball glove, fit for the garbage can. But Maniac knew at once this was Grayson's, the one he had played with all those years in the Miners. (Ch. 31)

By giving Maniac the old glove he used so many years ago when he played baseball for the Miners, Grayson is telling Maniac that he considers him a son.  Maniac treats the glove as if it were priceless, appreciating its value to Grayson.

Slowly, timidly, as though entering a shrine, the boy's fingers crept into it, flexed, curled the cracked leather, brought it back to shape, to life. (Ch. 31)

Grayson and Maniac have found each other, and have a comfortable and meaningful relationship.  Grayson wants Maniac to know what he means to him, and that is why he gives him the glove.  Grayson has not played baseball in a long time, and did not play baseball professionally for long, but it meant a lot to him when he did.  He wants Maniac to have the glove so he has a part of Grayson.

Unfortunately, Grayson dies not long after Christmas.  It is nice that Grayson got to have a family and a son for a short period of time, but it is sad that they only had a few months together and Maniac was alone again after that.  It is time for Maniac to go back to his other family, the Beales.