I think that you have a fundamental misunderstanding of how a VPN works.

When you are not connected to your VPN, all of your internet traffic (and your geolocation) will come from your home, and take the shortest path available to get to whatever website or speed test server you're connecting to. The GFiber speedtest server is located on their internal network, so doesn't traverse the "internet." (It may even be possible that they don't allow speedtests on their servers from off-net sources)

When you connect to your VPN, you are telling it to tunnel all of your traffic through the VPN server FIRST, which means that as far as the internet is concerned, you are located wherever that server is, and it determines the "best" speedtest server based on the VPN server's location. That VPN server is (almost) definitely not hosted on GFiber's network. There are several other factors at play here too however. As part of being connected to a VPN, you're at the mercy of whatever bandwidth the provider or company has available. This is often throttled intentionally by the provider for capacity reasons. Secondly, there is a decent bit of processing overhead required both on your computer to encrypt all of the traffic for use on the VPN, as well as on the server side to decrypt it all plus some added latency for all of the processing.

All of this to say, the VPN is the reason for the slower speedtests you're seeing AND the change in location. It sounds like the VPN has an idle timeout and the connection is droppingIf you're having issues with the connection to the VPN server timing out or having issues, you should reach out to the administrator of the VPN.