Gaelm has told you that in exchange for doing tasks for the Stable-masters of Bree, he will grant you the boon of being able to ride one of his steeds. Although lacking the power and stamina of his father's horses, Gaelm's horses are more tame and easier to ride.
'Have you come seeking a steed of your own? I may be able to help you. My father is teaching me to become a Horse-master in my own right someday, but currently the horses I breed lack the stamina his possess. However they are more tame than my father's horses, so you should be able to ride one, if you so wish.
'I will grant you this boon though in exchange for helping me. My father has made an arrangement with the Stable-masters of Bree. Instead of refunding their money for horses that have proven unusable in their trade, he has offered to do work for them. These tasks usually fall to me, but if you do some of these tasks for me, I will allow you to ride one of my steeds.
'The first task is to bring a sack of grain to Bill Rosewood at the stables near the West-gate of Bree. The grain is over to the east, by the silo.'
Pick up a sack of grain near the silo east of Gaelm at Hengstacer Farm. Once you have it, bring it to Bill Rosewood, the Stable-master who is found near the West-gate of Bree.
Bill Rosewood has asked you to draw water from the Boar Fountain in Bree and use it to muck out one of his stalls by the West Gate of Bree.
Speak to Bill Rosewood at the stables by the West Gate in Bree.
You mucked out one of Bill Rosewood's stalls, but he mentioned that Bert Goldenleaf, who maintains the stalls by the South Gate of Bree, may need the same service.
Draw some water from the Boar Fountain in Bree and use it to muck out one of Bert Goldenleaf's stalls. Bert Goldenleaf's stables can be found by the South Gate of Bree.
Speak to Bert Goldenleaf to see if there is anything else he would ask of you.
You also mucked out one of Bert Goldenleaf's stalls, as you did for Bill Rosewood.
Return to Gaelm at Hengstacer Farm in the Bree-fields.
You have done all that the Stable-masters of Bree have asked of you, so now you can return to Gaelm and see if he will now allow you to ride one of his tame mounts.