I have no idea what the mormons believe, I learned about it from the Scofield and Dake bibles. They don't teach about aliens.
The Mormons do not believe God is an alien, they believe he is an exalted human who originated on a different planet, and the goal in Mormonism is to achieve apotheosis and populate a previously uninhabited world with your spirit-children.
At any rate, the problem with the Gap Theory is that it contradicts a literal interpretation of Genesis 1, and opens the door to rejecting the central Christian belief that all things were created by God, unless one used it to somehow bridge the perceived disparity between Genesis 1 and Genesis 2 (the existence of which I doubt, but some people say these were separate creation narratives, which is an idea commonly associated with the five-source hypothesis regarding the Torah, where you have the Jahwist Source, the Priestly Source, the Elohist, the Deuteronomist, and the Redactor or Editor, which makes for interesting reading, but I subscribe to a semi two- source hypothesis, wherein Moses, although he was likely literate, composed the Torah either partially or entirely with a learned scribe, who then wrote the portions of Deuteronomy under direct divine inspiration that dealt with the repose of Moses, and subsequently assisted in the writing of Joshua, accounting for the stylistic and thematic similarity. Such a co-authorship idea would account for the slight differences in style in different pericopes, which in my view are not strong enough to suggest five separate sources being fused into one, and on the other hand, the five source hypothesis does not explain how a few Aramaic words got into the text, such as Genesis 15:1 and Numbers 23:10, but my thought in that case is these words were inserted by scribes over time, as Hebrew was replaced as the vernacular language of the Jews by Aramaic and related to a liturgical language, in the early stages of that process, when many people could still speak Hebrew, but Aramaic was the more common vernacular tongue, it makes sense that some Paleo-Hebrew phrases would become so obscure that many scribes would simply replace them with a more widely understood Aramaic replacement.
This doubtless occurred around the same time the Torah ceased to be written using the Paleo-Hebrew alphabet and instead with a subset of the Imperial Aramaic “Square Letters” alphabet in which it is still written to this day, which, with modifications to indicate vowells and cantillation marks, has also become the preferred alphabet of the Jewish community for writing Aramaic, Ladino and Yiddish, and is also used to write the neo-Hebrew official language of Israel, which was a remarkable accomplishment, in that the Israelis managed to revive a language the use of which had been confined to the synagogue, and in which no one had been conversant for at least 2,300 years, and this is promising for the attempts of the Coptic Orthodox Christians of Egypt to revive vernacular Coptic, and for the efforts of the Syriac Orthodox to arrest and reverse the decline of West Syriac Aramaic dialects such as Turoyo as the language of their people (and likewise for the efforts of the Assyrian Church of the East to ensure the Aramaic dialect spoken by a majority of their members, around 700,000, survives).
By the way, given the meticulous nature in which the Jews, whether Rabinnical or Karaite*, maintain the Torah today, using the system of Masorah to account for the occurrences of characters and provide error-detection (one of the first examples of an error correction code in mathematics, as is now widely used in computer science), it is remarkable to consider how at one time there were not only a mutliplicity of alternate readings of the Hebrew Bible, including the Torah, but that it would cease to be written in one alphabet and written exclusively in another alphabet, one of gentile origin. The Samaritan alphabet is somewhat more closely related to the Paleo-Hebrew alphabet, which suggests that the Samaritan claim to be the descendants of Ephraim and Manasseh is accurate, but one can also plainly see where they modified the text based on their beliefs about Mount Gerizim, which I suspect were as a result of the Northern Kingdom losing access to Jerusalem as a result of its separation from the Kingdom of Judah, and thus deciding to designate an alternate holy site, and since it was not until the liturgical reforms under the Holy Prophet Nehemiah and the High Priesthood of Ezra that widespread knowledge of the scriptures among the general population became a thing, something the Samaritans, at that point no longer a kingdom but a remnant people, would have likely emulated out of fear their people might otherwise be persuaded to convert, well, one can see how this could have happened in the Northern Kingdom.