Gap year after receiving good results worth it?

If my predicted grades aren't high enough for Russel group unis, but after receiving my results, I become eligible for a unconditional offer at a Russel group university for business management, should I take the gap year out or just go to the university I applied with my predicted grades (Aston).

It's honestly completely up to you. If you feel like you would much prefer to go to a Russel group uni then you can but keep in mind you are a year ahead of everyone there which could be awkward or strange to you if you aren't that used to situations like that.
Keep in mind if you are going to Uni for a stem subject then there are negative connotations with taking a year out due to 'slipping' of your skills in the subject. Also keep in mind you will be losing a year of your life but you can definitely use it to your benefit so it's not a complete sacrifice.

Many universities will consider applications with predictions below their standard offer. The RG “brand” is pretty much worthless in business - it means a university is research focused and doesn’t reflect anything about the quality of their teaching or the content of their courses. A placement year is worth far more for a business degree.

is there any particular universities that you’re interested in?
did you not use any of your 5 UCAS choices for aspirational choices?