why " geler"in le futur simple is "gèlera", rather than "gellera"

This is an interesting question Layra L.,

Language came in spoken form before written, so grammar is here to make sure the pronunciation is passed on correctly.

In cases where doubling the consonant would change the sound of the preceding vowel (making it short and sharp), an accent is used instead to preserve the original sound.

 

1. Verbs ending in -eler (except geler and peler):

e.g.

appeler ---> appellera 

rappele ---> rappellera 

épeler --->épellera

BUT 

geler ---> gèlera

peler ---> pèlera

2. Most verbs ending in -eter ( except acheter,  haleter)

e.g.

jeter ---> jettera 

projeter --->projettera 

feuilleter ---> feuillettera

BUT 

acheter ---> achètera

haleter ---> halètera

Hope this helps!