Am trying to clear a thread up by deleting posts within it

Every time I try I get a 504 gateway timeout error?

 

Gateway Timeout

The gateway did not receive a timely response from the upstream server or application.

Additionally, a 504 Gateway Timeout error was encountered while trying to use an ErrorDocument to handle the request.

Would this be a server thing or software thing?

This is a server thing. Are you on a shared plan or are you runing a vps/dedicated box?

How big is the topic in question too?

Essentially the server timed out which could be due to many different factors.

Try temp disabling your third party stuff and then deleting.  

I just had to delete many pages of posts with broken attachments, and it was fine.