Portal just means gateway, and a variety of more specific versions of "gateway" or "entrance" as seen in the Merriam Webster definition. I can't track down the origins modern association with Sci-Fi, partially because a lot of writing about the history of Sci-Fi uses the word "portal" in that sense when describing works that used other terminology. However, the Science Fiction definition of portal is just another specific extension of the original word in the sense of a gateway between two places.

What I can do, is explain a bit of the history of the word "portal." Portal, also frequently spelled porttol, first appeared in Middle English in the late 14th Century, with basically the same modern meaning of "gateway." This was right around the same time that the Bible was first translated into English by John Wycliffe, though the Wycliffe Bible doesn't use it in this passage, instead saying:

Ye princes, take up your gates, and ye everlasting gates, be ye raised; and the king of glory shall enter. (Lift up your heads, ye gates, yea, ye everlasting gates, be ye raised up; and the King of glory shall enter in.)

In fact, on further investigation, I'm wondering what translation you were reading, because none of the common translation I checked (KJV, NRSV, NIV, CEV, CJB, NKJV, even 1599 Geneva) used the word "portal" in this passage from Psalms. All of them uniformly say "gate." From a translation perspective, there's no real difference, but you seem to be reading a different version.

Portal was borrowed from an Old French word of with the same spelling and meaning, which in turn was related to the Latin word porta. In Latin, porta also meant gateway, but the same word was ultimately the root of several other related terms like "portico," another type of gate or opening, and "porch," as in the area immediately outside of the gate or door. Those derivatives developed through medieval Latin use of porta.