I read this as being that he has tried running both Steam and GOG versions, and neither work for him.I suggest you contact GOG support. I fully expect that they will be as unhelpful as the generic troubleshooting steps that already failed you, but contacting them will make them aware that the game does not necessarily work out-of-the-box on a supposedly supported platform. Additionally, should you decide to seek a refund on the GOG version due to inability to use it, they will insist that you first humor their useless attempts at troubleshooting. Attach it instead. The forum permits common image types. They will not appear inline, but will be archived with the post, which is better long term anyway.

If you are not specifically attached to the DOSBox experience, I suggest you try OpenXCom. It is a modern reimplementation of XCom, has numerous quality of life improvements, and runs natively on modern platforms. It will require original XCom data (so your purchase is not a waste), but will not reuse any of the original code.