I saw a video in which someone used GC Electronics Type 44 Non-Contaminating Heat Sink Compound (Part No. 10-8118) to hold the resistor for heating up the J-Head extruder head in place.
Is that a good way to attach it so it heats up the aluminum block?
Nope, not adequate. When heating at maximum power, you should expect the resistor to be 50-100C hotter than the hot block. (It must be hotter than the block in order to transfer heat to the block.) GC 10-8118 is only rated for 200C, but the service conditions at the power resistor may run up to 300-400C depending on power output and desired hot end temp.
For that matter, you may want to question whether the power resistor itself is rated for the >300C temps it will see in normal service. It's probably not. Most modern hot end designs use steel-jacketed heater cartridges instead of power resistors because they can handle higher temps and are, you know, actually designed to be used as heaters. The power resistors used in older hot end designs are generally being operated way past their design ratings. It's kind of amazing they work at all.