According to the wiki, the GBE of the Sun is “
5.693E+41 J or
136.066 Quettatons of TNT”. If we are going by this, to force a star like our Sun into its very final red giant phase, you must at the very least overcome the gravitational binding force holding the outer layers to the core, requiring about 10–20% of its total gravitational binding energy which is at least 5.7 x10^40 J to 1.1 X 10^41 J Low star level.
If it was the initial translation we got that mentioned
synchrotron radiation specifically it would mean that the star is at least a massive star that went supernova or in its very final stage of red supergiant phase which is
4b or 4b over time. I don't agree with this interpretation.
Personally, I believe this feat is at least Low Star Level, as it required overcoming the gravitational binding energy holding the Sun's outer layers to its core. Since Momoshiki accomplished this in his base form, it stands to reason that his Ap in his fused state, and specifically his final attack against Boruto which is stated to be his greatest technique ever would surpass the power of the technique used to achieve this feat in base.