Car clunk at low gear change and transmission light is on, would changing gearbox oil fix this problem ?

I have had car for 9 months, it has had around £2,200 on repairs on it since then. now chemical test on water in the coolant shows bad head gasket, a different garage charged £950 for hg several months ago but found out they only did water pump, they are refusing to fix it now that I informed them it has gone. so I have to pay out for that to be done now , it runs well but have to top up water once a week, no water/oil mixing, no power loss or smoke

I also have my transmission light come on often ( has done since buying the car but didn't realize what it was until several weeks later, only came on a few days after buying the car ) there is also a clunk noise during low gear change and when on idle and taking foot off clutch pedal.

Car goes into every gear brilliantly without delay. my friend said by changing the gearbox oil ( old oil dirty and had tiny little bits of metal in it ) that it should fix the problem.

is he right ?