<p>I am making a bed for my 3D printer. I have bought a silicon heater (31x31 cm) and I want to glue it to my custom aluminum bed. The tape that it had from factory was bad, so I removed it. I want to glue it to the aluminum and I don't know what type of adhesive to use, I was thinking gasket glue with silicon, but I think that it will have bad thermal conductivity. I found <a href="https://www.annapol.eu/product_info.php?products_id=137997&amp;language=en" rel="nofollow noreferrer">this product, a silicon based,  heat transferring paste</a>, but I think that it will not stick good. What is a good adhesive for this purpose?</p>

Heat Transfer PAste will not work as a gluing agent. What you need is a high-temperature glue that bonds Aluminium and a silicone rubber. The benchmark temperature that the glue needs to withstand is about 100 °C or approximately 200 °F.
McMaster-Carr
allowed me to search for glue-properties and suggests among others contact adhesives, which are cans or tubes with a very viscous glue. You let that pre-cure on the items and then push the two together, resulting in very strong bonds. One of the items they suggest is
3M 2262
for which McMaster-Carr lists 230 °F as the max temperature, so just in the
safe
range. However, a 32 oz. can for almost 100 $ is surely overkill, the
3M manufacturer catalog
lists only 1 qt and 5 gal as the available packaging sizes, so, unless you build printers for a living or know someone who uses this industrially, this is not an option.
The same catalog also lists the
"3M™ 200MP& 300LSE"
series with temperatures of 148 °C (300 °F) to 204 °C (400 °F). That is enough to print anything on. And it's the same glue that' used on my Build-Tak replacement surfaces to stick them to the Aluminium bed, so it could be suitable for gluing the heater to the bed on the other side too. However, I wish you good luck to find a reseller that sells you a fitting amount. Industrial packaging is available, but you'll need to find a reseller that offers a somewhat fitting size for you - I have seen Amazon listings for both types in sizes that have only small waste for most common heaters.
I am not affiliated with 3M or McMaster-Carr, but I have very good experience with 300LSE.