Is it possible to make a PGP message that anyone can decrypt?
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You mentioned PGP, we will assume that you want some obfuscation of your message at rest.
Now the issue becomes is when people need some kind of unlocking mechanism to get to original text from your obfuscated message.
Such unlocking mechanism is called a key or an algorithm.
You will need to better elucidate how you want that algorithm/private key to be distributed to your audience.
Until your requirements are further refined, your problem statement shall remain not impossible yet quite easily solvable but not in ways you expected. Read on.
Our recommendation (given your expertise) is a substitution cipher, like ROT13 to be applied exactly twice.
You will find this ROT26 to be a suitable answer in being able to address all of your audience.
Sometimes they want to send a plain text message, except have the receiving tool still unconditionally "decrypt" it and not, for example, give an error that it is not a PGP message.
Maybe it's a bit of a stretch.
But it's not too dissimilar to compression formats that have non compressed blocks, e.g. deflate's non compressed blocks or zstandard raw blocks. Their purpose is more for incompressible data or very small data, but can be (mis)used to have uncompressed data that can be "decompressed" by a standard tool the same as any normal compressed data.