Hi @rmdevelop ,
Could you please share a bit more about the relationship between the Manufacturers List and Model List?
How do you set up the relationship between the Manufacturers List and Model List? Using LookUp type column?
Based on the issue that you mentioned, I think there is something wrong with your operation. You could not use the Visible property of the Gallery control to hide the Gallery Items.
I assume that you set up a LookUp field in your Model List, to reference specific Manufacturers Name from your Manufacturers List, is it right? The relationship may look as below:

I have made a test on my side, please take a try with the following workaround:
Set the Items property of the Manufacturer Gallery to following:
Filter(
'Manufacturer List',
!IsBlank(LookUp('Model List', Manufacturer_Name.Value = 'Manufacturer Name'))
)
or
Filter(
'Manufacturer List',
!IsEmpty(Filter('Model List', Manufacturer_Name.Value = 'Manufacturer Name'))
)
Note: The Manufacturer_Name column (Lookup type) in your 'Model List' reference values from the 'Manufacturer Name' column in your 'Manufacturer List'
You could also consider try the following formula:
Filter(
'Manufacturer List',
'Manufacturer Name' in Concat('Model List', Manufacturer_Name.Value & ";")
)
Please take a try with above solution, check if the issue is solved.
Best regards,