15 meter zinc gutter, flex enough to bend a little (0.5 degree)?
Galvanized steel will easily take a small strain like that. The normal yield strength of steel is not reached until total strain is 0.5% strain. That is ,it would need to be held in place to prevent "springback" at 0.5 strain.  However , 0.2 % is permanent strain ( no springback). Or , extreme case ,you bend it slightly ( I doubt that will be necessary).  If you want a 6 m ( about 20 ft) length with no seam you will need to use aluminum , not galvanized steel.