This is an important and complex area. Here's what I know from helping my own uncle through this:
The Gurkha Pension Scheme (GPS) was the main pension for those who served before 1997. The amount depends on years of service and rank. Since the 2007 Act, pre-1997 veterans who settled in the UK can also access certain UK state benefits, but the GPS was not automatically equalised with British Army pensions at the time.
What to do:
1. Contact the Veterans UK helpline (part of MOD) — they can provide a pension breakdown
2. Contact the Gurkha Welfare Trust (gwt.org.uk) — they have specialist caseworkers who help with exactly this situation, for free
3. Get independent legal advice from a solicitor specialising in Gurkha cases — several are listed on NBD
The key question is whether your father is entitled to any enhancement or back-payment under subsequent legal rulings. A solicitor who specialises in this will know immediately.