((climberjoe)) refluxy baby is enough to bring all of us to tears and sorrow, I feel sad just hearing about your LO!
Honestly, if your babe is in that much pain even drinking water, I would try other meds asap. No constipation, potential relief. My dd will be on meds until around 18 months simply because her reflux is related to a floppy larynx, and thats when the latter will settle down and harden - hence reflux should settle too. Others vary, often until around a year when they're more upright and mobile and on solids. But your GP is right, its often around six months too - when weaning can help (not always, but often). Solids stay down much better than liquids do.
Gaviscon, for us, was not helpful - I'm sure it can be for others. Interestingly, it was not much use to me when pg either - I took Ranitidine, much more effective and saved me from permanent gullet damage!
I totally understand the shock and getting head around change in 'wrong' direction feed-wise....dd's reflux did'nt kick in till she was 6 weeks old, and it did feel like going backwards even at that age. Like any diagnosis, it takes time to accept and adjust to. If you're not an anti-hug MNer, please take one large ((())) for yourself from me and my now much happier dd: we know what you're facing, and it does get better - fat lot of comfort that is now, of course, but true!