What should I use to bond an aluminium bracket to a GRP panel?

Bonding two materials that move at different rates is exactly where a proper structural adhesive earns its place over a straight laminating resin. Metals and GRP expand and contract by different amounts with temperature, so the joint needs an adhesive with enough toughness to absorb that movement without cracking, which many structural adhesives are formulated to do. Surface preparation matters as much as the product, so abrade both faces to a clean matt key, degrease thoroughly, and do not touch the prepared surfaces before bonding. A gap-filling toughened grade also copes with the slightly uneven fit between a bracket and a panel better than a thin, rigid glue line. Since the right choice depends on the load, the metal and the service temperature, tell us the detail and we can match the adhesive to the job, or look at materials testing if it is properly structural.