The process starts with a 20-30 minute general phone interview. If you succeed you will be invited for a 1:1 interview. Beware the 45 minute long competency based phone interview. It's awful without human interactions and with lots of brainless interview questions.
Before the 1:1 you have to complete a 45 minute long OAKs test. Depending from the role you can get a few skills tested, eg.: numerical estimation, mechanical comprehension and following instructions. This is supervised by Randstad.
If you succeed (30% of applicants) you get the 1:1 interview. First you will hear about the job. Then you discuss your CV in high detail. Expect some basic interview questions: what do you know about the company, what are your motivations etc. After these you will have the competency based interview. You will be asked 1 question on each GSK expectations (there are 6). You have to reply using the STAR format. If you are still hanging there you get a few short questions as well about your behaviour.
Don't forget to always show interest and smile. :) Ask the person about the next step. Good luck!