Answer
God the Father is the God of God in Psalms 45:8.
We can understand this from other parts of the Scripture.
Explanation
I am not a Jew who followed the Tanakh and ended up in the New Testament as a Christian. I am a Christian, which means one who follows Christ and His Apostles, and I understand the Old Testament through the New Testament.
The New Testament is crystal clear in its teachings.
For example, in Hebrews 1:10-12, we see God the Father calling His Son Jesus Christ as Yahweh who created the heavens and the earth:
“God spoke” (Heb 1:1) “but as to the Son” (verse 8) “And, "You,
Lord (in the Old Testament, this is Yahweh), at the beginning founded the earth, and the heavens are works of Your hands.
They will vanish away, but You will continue; and they will all become
old, like a garment, and You shall fold them up like a covering, and
they shall be changed. But You are the same, and Your years shall not
fail." (Heb 1:10-12).
[The inspired author of Hebrews depicts God the Father as quoting Psalms 102:25-27 which talks about Yahweh who created the heavens and the earth; but God the Father applies this to Jesus Christ]
Hence it is not difficult to understand that the same God addresses His own Son, Jesus Christ:
“Your throne, O God, is forever and ever” (verse 8).
So, the One who speaks here is God the Father. He calls His own Son as God.
Like Father like Son.
He calls Jesus, God twice; once as we saw above (in verse 8) and the second in verse 9:
“You have loved righteousness and have hated wickedness; because of
this has anointed You O God (in the vocative), the God of You,
with (the) oil of exultation above the companions of You” (Biblehub).
For better clarity, I rearrange the above in a better rendering:
“You have loved righteousness and have hated wickedness; because of
this, the God of You, has anointed You, O God (in the
vocative), with (the) oil of exultation above the companions of You”.
Conclusion
Jesus is referred to as the Creator God (precisely Yahweh) in the New Testament. So, definitely, His God (God of God) is none other than God the Father.
Nowhere is a son different from his father! If father is a human then definitely his son also is human.
So, if the heavenly Father is God, then definitely His “monogenes” Son is also God, just as God the Father proves in the Scriptures.
No wonder, “God” is the English translation of the Greek “Theos” which in turn is the Greek translation of the Hebrew “Elohim” which is a plural word used in singular most of the time.