Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
First, I do want to note that we do tend to use this medication every 8-12 hours. Therefore, if you have only given the Gabapentin once, then we may need to consider a second dose to get her comfortable here.
Afterwards, since the ER vet already ruled out a gut blockage for her, we'd want to try and see if we can help her pass feces. Further to milk, you can certainly add a GI lubricant to her food. Food grade mineral oil can be used or we also can use a few milliliters of Miralax or Lactulose. And again, its best to add these to food as opposed to giving directly to reduce the risk of aspiration from direct administration.
Finally, if she has been having fecal passage issues, then you can consider adding fiber (ie a spoonful of canned pumpkin, all bran, etc) to her food. This can just help restore fecal regularity and keep things moving for her.
All the best for Maxie,