but in the end GPG says "Untrusted signature - This signature is not to be trusted"
This just means that you haven't manually set the key you imported as "trusted". The signature matches, which is the important part, but... what does that mean? What if you used a random/fake key and it matched? The software doesn't know if the key is really from ThomasV, so that's why they give this warning.
I think you can set it as trusted by right clicking it and choosing whatever option is there (depends on the software you are using).