GFCI keeps tripping from garbage disposal
Assuming that there is no other electric device connected to that GFCI - also not via a downstream/"load" outlet:

There might be a safety issue with that disposal, so taking care of the danger of electricity is mandatory. 

Connecting the disposal to another GFCI outlet via an extension cable could disclose whether the disposal or the GFCI is defect.

The disposal or metal sink or other conducting material in connection with the disposal should not be touched, i.e. it should be switched on and the last step should be to plug in the extension cord into another GFCI outlet. 
To stop this test, the first step should be to unconnect the plug of the extension cord. 

After some years, the electric isolation of the disposal could get worse due to humidity, dust, vibrations, grease/oil, acid etc.

The test button of the GFCI might show no problem, but that button can not test an over-sensitivity.