I know that Wemod is a very popular one. But Wemod doesn't support any GOG games. Is there any other safe trainer that supports GOG versions of games?

Coromon is the only Pokemon-style game I know of on GOG.

Cheat Happens trainers usually work fine with GOG games. Both their main online trainer system, and also their older stand alone downloadable trainers.
Occasionally, there are platform specific issues with trainers, and yes, I've been caught with that. So post the bug report info in the relevant trainer thread.

It's not that WeMod does not support GOG, it actually does, but barely anyone makes trainers for the GOG versions of most games on it. Out of the ones I currently have installed, there is only 1 game that has a GOG cheat table - Vikings: Wolves of Midgard.

I assume it'd work fine most of the time if the cheats from the Steam version could be used on the GOG version through WeMod, but it does not allow that as you can only use it on games that you have installed.

Your best bet is probably finding custom CheatEngine tables.
Or try Cheat Happens, as already mentioned, but you have to get lucky over there and hope the trainer for the game you want is available for free, as a ton of them are gated behind paid membership.

Even CheatEngine is paywalled. The more recent releases of CheatEngine require you to subscribe to the CheatEngine Patreon.
.CT files made for older releases of CheatEngine won't work on more recent versions, if they call on CE functions that have been depreciated, and of course vice versa also applies.

Their homepage links to a Git repository, so I wouldn't exactly call it a paywall, but I see the changelogs talk about "Patreon" vs "public" versions, and their Patreon descriptions sound like some of their builds contain ads. :-/ Stable versions of mods and such tools would also be good to preserve. Good old trainers.

Some of my earliest PC gaming memories are hitting a hotkey to set some memory to null/noop and watching (single-player, offline) virtual worlds incorporate the customization. And what happens if I set the cooldown time to 0, or change a greater-than to a less-than in some formula? Ah, marvelous.

Most games are so simple that if you have a hex editor that can do a memory watch or hook onto a process that you're basically golden.

Making that persistent might be harder, but it can be done. That's how the Game Genie and Action replay worked.

Variable goes down by one, find variable and put it back up to 3, for example.

And don't even get me started on what you can do if you reverse the polarity of the neutron flow!

Almost all trainers support the GOG version.
Cheat engine, Wemod, others.
Just select the executable manually and it will work.

That would be amazing if publishers started including cheat engines with their own games. I mean, some do in the form of console commands I guess.

Pfft, good luck with that. What once was maybe included as a bonus feature/rewards for playing the game, is now all but "DLCs" and micro-transactions. Hell, when publishers like Capcom go introducing DRM to games that have been out for a number of years already to stop any kind of modding, you know there is no chance of that happening anymore. It's all about pure greed now. Capitalism: ain't it wonderful? =/

I could be your personal trainer for GoG games..

(Hmmm can't find that video that's like 5 years old, of an esports trainer satire with a coach behind a guy at a computer yelling CLICK FASTER! CLICK LIKE YOUR LIFE DEPENDS ON IT! MY GRANDMA CAN CLICK FASTER THAN YOU type of stupidity).

Is it safe to download? Like, no malware or stuff?

That's what CheatEngine was made to do. So I'm not sure why you would not just use that.

Because I'm an old fart who existed long before cheat engine did.