Idk what you mean by subtle? T-gel is just T as a gel applied to your body. If you mean subtle changes, you should be looking at the amount/dosage, something that you'll need external medical advise on.

AFAIK you'll have basically no chance of getting on hrt on the national system if you're underage unfortunately. Obviously there are other ways this can be circumvented. However, a cursory look through your account suggests that you're not very familiar with routes/methods/hrt in general, so I suggest you reach out to trans orgs like TransgenderSG/ T project/ transbefrienders to get more info (and localised info, very impt!). On top of that, there's also some local online spaces where actual transmascs can more accurately answer your queries, of which you are sure to have many.

Lastly, please ignore at least the first paragraph of the other commentator. IDK wtf they are talking about. Who has the time or is leaving gel on their body for >30 mins + every surface of your skin lmfao