Some sort of breakdown agent like the RAC would, surely.
Alternatively, save a few bob and buy one and fit it yourself - I'll qualify that by saying I know nothing about the GS, as I don't know where the battery is located, but on many bikes its under the seat - 15 minute job. Hth.
Jump leads to get it started (don't run the donor bike car engine) then ride the bike to where you can fit a new battery. Otherwise call an auto electrician.
The starter pulls a huge current that can overstress the donor alternator or even the bike voltage regulator.
The extreme power given by the car generator is minimal compared to the current punched by the battery. So it's not adding much value so no point taking the risk.
The donor vehicle will surge up to maximum output when the bike is cranked over, once the bike starts the bike alternator also starts to run at full power thus resulting in a power surge capable of frying the ecu on both vehicles.
Doesn't always happen but I have seen the results of this more than once when I owned a car and motorcycle workshop.
Always have the donor vehicle with the engine off and wait a minute or two after the bike has started before removing the jump leads.
The starter pulls a huge current that can overstress the donor alternator or even the bike voltage regulator.
The extreme power given by the car generator is minimal compared to the current punched by the battery. So it's not adding much value so no point taking the risk.