Hello there, @userkeith-hume.

QuickBooks Online integrates with multiple applications as long as it's supported and available from the app center. To check and verify if Garage Data System (GDS) is supported, I encourage visiting our app center.

You can also contact GDS support. This way, you can ask them if they can integrate into QuickBooks and help you with the integration process.

Just in case you need references while working with QuickBooks in the future, feel free to read our help articles page: Help article.

Get back to me if you have any other questions. I'm a few clicks away to help. Have a good day!

@userkeith-hume 

I checked it supports Sage, Xero and QuickBooks. You can call them directly (01482.33073) and ask their assistance to migrate connection from Sage to QB Online.

https://quickbooks.grsm.io/UK

You may need to buy a 3rd party conversion service if you need to migrate attachments from Sage to QB Online. 

@userkeith-hume 
Hi Keith, I know you posted this a while ago but I'm interested to find out if you did ever manage to get GDS to integrate with QB?  If so did work for you and have you had any issues.
My client made the move last July and the first year has been plagued with problems, some of which suddenly appeared after not being an issue before.  I have some work arounds for some of the issues but it's proving to be very time consuming - I'd be interested to hear of anyone else's experiences either good or bad.
Thank you.

Hi Sam

Unfortunately I didn't get anywhere as I was just 'passed around' to be honest by both GDS and Quickbooks!

to be honest Keith I think that was probably for the best.  We've been using it for year now and new problems keep suddenly appearing! 
Do you use GDS with Sage still and is that working fine?