Here's what the TfL journey planner recommends for Gipsy Hill to Blackheath.
Wouldn't it be quicker just to take the train direct from Gipsy Hill to
London Bridge instead of all that mucking about on different bus routes?

09:19 From Gipsy Hill (SE19)
Take Bus 3 towards Oxford Circus to Thurlow Park Road (SE21)
Buses every: 7-8 minutes
Max journey time:16 minutes.

From Thurlow Park Road (SE21)
Walk to West Dulwich.
Transfer time: 4 minutes

09:45 From West Dulwich Station
Take Bus P13 towards Streatham Bus Garage/Ice Arena to Tulse Hill Station

Change on foot. Using . Transfer time: 4 minutes

10:01 From Tulse Hill
Take OO 32442 South Central towards London Bridge to London Bridge

Change on foot. Using . Transfer time: 4 minutes

10:25 From London Bridge
Take OO 4999 South East Trains towards Dartford
or OO 4609 South East Trains towards Charlton to Blackheath

10:18 arrival at Blackheath

Max. Journey Time: 01:18; Interchanges: 3
Price: £ 2.0 for adults, £ 0.80 for children