ATG (snot) tape

Used to attach gels to gel frames. In the short term, it can easily be rubbed off of the frame with no residue.

From the manufacturer:

Scotch® ATG Adhesive Transfer Tape 924 quickly and permanently bonds a wide variety of materials including uncoated and coated paper, card stock, paper board, matting, lightweight fabrics, foils, flexible plastics and foam board. Offering high adhesion and excellent UV resistance, it easily bonds to difficult, varied and textured surfaces, including areas with high humidity and sunlight.