GPG is a tool to solve the technical challenges of authentication, signing, and encryption, and GPG is secure in the technical sense. However, security is not just a technical problem, there's also the issue of trust.
How do you define who to trust? This is not a question that can be answered by the tools, but rather something that the users of the system, in particular, the relying parties need to decid for themselves. So you need to ask the relying parties who they'll trust to attest to your claim.
The trust made on a system does not necessarily translate to trust in the person using the system. Also, the trust on the system to be usable to make a particular claim doesn't necessarily cover all possible claims.
For a more concrete example, a GPG signature by itself cannot be used to make claims about time as you could've made the signature at any time before or after the time you claimed you made the signature (the timestamp in a GPG signature is easily forged by the author). To prove time to a third party, you may need another system like a timestamping server, publishing hash, or some knowledge that could not have been acquired before a particular time to make certain claims on time and that you actually have a certain data before a certain time. An example of where this could be relevant is for making claims that you authored a certain material by publishing proof of timestamp, GPG alone isn't sufficient to do that, although GPG can be used as part of a system to prove that.
Also, legally speaking, a court may decide that certain evidence are inadmissible for certain scenario just because they don't understand how the proof works, even though technically it should be sufficient to prove your claim. You may be able to fight the long battle for a judicial review to convince the court that your proof works, but it could've been massively easier to supplant it with methods that have already well established legal precedents rather than doing something novel.