I just finished OCR Gateway last month.
I feel like two months might be a little ambitious especially if you're going to school and doing all of your other subjects. At my school, science had the most periods and the department struggled to finish regardless. I think my class was the only one that went over all the modules properly.
For me personally, I found Chemistry the easiest.
Biology I did last year and didn't take seriously. I got an A* in that but I did better in B4, B5, B6 than B1, B2, B3 even though I thought it would be the other way round. I would advise you do a lot of past papers so you know how you perform in the exam because sometimes you can learn content and the books sometimes disagree on the wording and OCR want a specific wording.
Chemistry was good for the most part and once I got my head around C5 and C6, they were straight forward.
Physics was a bit iffy for me. I didn't understand P3 until I did P5. Then I didn't understand P5 that well and P6, I went over a lot, but didn't properly understand until the day of the exam. For me it was difficult to grasp the concept until I learnt how and why the thing worked like that e g. transistors. The same went for a few other people in my school so it might be the same for you.
From skimming through the AQA specification, I noticed a few differences in content from OCR. They don't include SUVAT equations, transistors, resistors, logic gates, horizontal and vertical projectiles and momentum during crashes and rocket launches. I don't think these are impossible to learn by any means but there might be a learning curve when trying to grasp the things.
I'm not sure if these things are still relevant because we're doing different specifications but I hope they are. I hear you guys are doing an extra unit instead of a controlled assssment too and have to learn the equations.
Some good youtube channels to check put are
Streetly Science, Westlands Science, GCSEChemistryTeacher & DrPhysicsA. You can watch these to get an idea of the content and use them if you decide to do it.