It's a very simple process, OP. I've done it plenty of times before. Allow me to guide you through it.
First of all, you're going to make a dummy email account and send an email to GOG's support staff saying you have GOG Wallet funds you want converted to Steam Wallet funds. Then you're going to make a DIFFERENT dummy email and send an email to Steam's support staff saying you have a Steam gift card you want converted to GOG Wallet funds. Make sure you do this on a weekend, as it'll ensure that Steam gets to you first, since, as everybody knows, every third Monday, second Tuesday, and second Thursday of the month is a Polish holiday, on which the staff engages in pierogi fights. When Steam emails you back and tells you that they can't do that, screenshot the message, which will say something along the lines of "these funds are only usable on Steam". By this time, GOG will have gotten around to you telling you something along the lines of "these funds can only be used on the GOG store", which you will also screenshot. Send the screenshot of GOG's support to Steam's support, saying that you contacted GOG and they said that the funds are only usable on GOG's store. Send the screenshot of Steam's support to GOG, saying that you contacted Steam and they said that the funds are only usable on Steam. Continue to goad both sides in such a manner until Steam and GOG eventually set their lawyers on one another. Try something like "Steam said you guys are nothing but pirates!" or "GOG said you guys treat your customers like crap!" or "Pierogis suck!" or something similarly inflammatory.
At this point, GOG's lawyers and Steam's lawyers will be in a heated battle, whereupon you'll head over to superconvertomatic76.com, which, despite its appearance, is actually the hub of an underground gambling ring. You're going to go to the "convert-a-tron" page and in the area where it says "convert one website to another", select "superconvertomatic76.com" in the first box and "johnnygogingsjohnnygambilngextravagoganza.notillegal.net" in the second one, then click "convert". You'll be taken to the gambling area, whereupon you're going to take at least $500 and put it on GOG. Most people assume Steam would win the legal battles due to having far more money and, consequently, more expensive lawyers, but few take into the account that none can defeat the Polish in a pierogi battle. The judge will throw out the case but decide not to press any charges for wasting the court's time on account of having the most delicious pierogis he's ever had in his life, declaring GOG the victor in terms of pierogis, and you'll win the bet.
Now just take all the money from your bet, buy a cow or other large animal of your choice, sacrifice it to summon Yog-Sothoth, and ask him to convert your Steam Wallet funds to GOG Wallet ones!
Hope this helps, let me know if you have any questions.