The answer is, not as far as the public knows. If it can be broken, it's probably costly to do. Using GPG on a PC is like putting a bank vault door on your house. Someone who wants in is probably going to go through the window or the wall or the roof, rather than attacking the front door. Here I will list some of the ways you can be compromised without breaking the cipher, and suggest precautions you can take.
You can contact the person, look up the key on a website, or check a signature using an external GPG key manager. If your adversary controls your Internet access or the recipient's, the keyid you see on a website might not be the actual key. Altering web traffic in transit is possible and has been done in real attacks. Hacking of social media accounts is quite common.
Many programs have automatic update features. Some automatic updaters have weak security if any. These can be used by an adversary to "update" your computer with malware.
The most paranoid approach is to buy a cheap laptop and use it only for Confidant Mail. Use a flash drive to load the software, and encrypt the laptop's hard drive. Do not access the web from that machine or install any software except Confidant Mail and disk encryption software. Turn off automatic updates, turn off the built-in network services, and work behind a firewall. If you think some powerful organization is out to get you, this is a good idea.
The second most paranoid approach is to use virtual machines with VirtualBox or similar software. Create two of them, one for web access, and one for Confidant Mail. Do not access the web from the machine itself. This provides quite a bit of protection, but a custom-made attack against a high-value target could probably break out of the virtual machine.
If the computer is encrypted, the intruder can plant either hardware or bootloader software that will save or transmit the passphrase the next time you start the machine. If the passphrase is saved, the intruder has to come back to get it. A more sophisticated hack would transmit the passphrase, allowing the attacker to decrypt the disk image he made.
You will have to figure out your own custom means of detecting physical intrusion, and they are likely not foolproof. If you find that your encrypted computer has been tampered with, booting it up is the worst thing you can do. A better approach is to use a USB hard drive caddy, and a virtual machine with no network, to restore your data from the old hard drive onto a new machine. However, this requires some technical skill and a lot of time.
You should therefore keep a regular backup on an encrypted external hard drive or flash drive. In an emergency, go buy a new computer and restore your data onto it.
Messages are received in wx.RichTextCtrl XML format, which can have embedded images and complex formatting. There might be an exploitable bug somewhere in the rendering code, so the program has an [Open Txt] button. This displays the text-only content without the formatting and images, making an exploit less likely. If you receive a message from an untrusted user, either delete it or use the text-only option.
SSL libraries and even GPG might also have exploitable bugs. There is not much I can do about those. The client does not accept incoming connections, but someone might intercept an outgoing connection and send an evil payload back to you.
Confidant Mail has an update notification system, but does not automatically update itself. The notification system is not cryptographically secure, and could be spoofed by anyone who controls your DNS. If a URL appears in the update notification line, do not go to that page. Check the official site for a new version.
Someone who wants to target you might start by hacking one of your less paranoid associates. If the attacker gets your friend's key, the attacker can send you a message that looks like it came from your friend, and trick you into running something nasty.
Hotel Wi-Fi is just as bad as hotel physical security. Many of these systems put multiple users on the same network, so anyone else in the hotel can see your computer and attack any ports you might have open. Some are vulnerable to ARP, DNS, or DHCP spoofing, all of which permit a man-in-the-middle attack on your web access to implant malware. Failing that, the attacker could unplug the hotel's access point and put up his own in its place. Using a cellular data card is usually safer, although those can be attacked with a "Stingray" device.
Coffee shop and other public Wi-Fi is just as insecure as hotel Wi-Fi. Hotels are easier to target because you stay in one place for days. If you regularly work in the same coffee shop, you can be targeted there.
GPG has a subkey feature, in which your master key can have separate encryption and signing subkeys. Senders will use the encryption subkey instead of your master key. You can change the encryption subkey periodically, and destroy the old ones, while your public key ID remains the same.
As of version 0.26, subkey rotation is automated. Click Actions/Rotate Encryption Subkey to trigger it. The detailed explanation of the forward secrecy feature is here.