It was indeed gelatine, and faded amazingly quickly. In the 60s, gel really was gelatine, but plastics became popular in the 70s. The first 'proper' man-made gel was an acetate, quite thick and it hardened quickly - especially in the more saturated colours - going almost transparent in the centre of the beam. Brands were Cinemoid (Strand) in the UK, and Rosco produced similar stuff in the US. Polycarbonate or polyester is the current material used by most manufacturers - polyester being more heatproof, so better for high temp use.

Thanks to all...a lot of good info.
Jeff