Here are two references:

Frequency, timing and outcome of gastrostomy tubes for amyotrophic lateral sclerosis/motor neurone disease--a record linkage study from the Scottish Motor Neurone Disease Register
Frequency, timing and outcome of gastrostomy tubes for amyotrophic lateral sclerosis/motor neurone disease--a record linkage study from the Scottish Motor Neurone Disease Register - PubMed
" . . . The 1 month mortality after gastrostomy was 25% . . . "

[Neuroprotective therapy for amyotrophic lateral sclerosis (ALS)]
[Neuroprotective therapy for amyotrophic lateral sclerosis (ALS)] - PubMed
" . . . but the effects were insignificant in the Japanese nation-wide trial."

Regarding riluzole in Amyotrophic Lateral Sclerosis, regarded as +three moon shift in survival medians; survival term improvement from gastrostomy is unclear.