Gravity Falls
That's all from the Gravity Falls, 1s2s exactly.

1.But I got proof this time, by gummity! - What does "gummity" means?
2.It chopped my boat up to smitheroons! - "smitheroons" ?
3.It shim-shammed over to scuttltebutt island! - "shim-shammed"?
4.My ex-wife still misses me, but her aim is getting better! - What does it mean, exactly? She's hunting him with a rifle or just He ditched her and feel great about it?
5.In retrospect, it seems a bit contrived. - It seems a bit complex ?
6.You just don't know the lengths us old-timers go through for a little quality time with our family. - It means: "It's hard to bond with your family when you're an old man" ??
7.I created a homicidal pterodactyl-tron. - "tron"?
8.I had a run-in with the lake police! - He was fleeing from them?
9.an old man:"Come quick before he scramdoodles away!" ... Mabel:"He did use the word "scrapdoodle". scramdoodles, scrapdoodle - ??

Question

Gravity Falls
That's all from the Gravity Falls, 1s2s exactly.

1.But I got proof this time, by gummity! - What does "gummity" means?
2.It chopped my boat up to smitheroons! -         "smitheroons" ?
3.It shim-shammed over to scuttltebutt island! - "shim-shammed"?
4.My ex-wife still misses me, but her aim is getting better! -  What does it mean, exactly? She's hunting him with a rifle or just He ditched her and feel great about it?
5.In retrospect, it seems a bit contrived. - It seems a bit complex ?
6.You just don't know the lengths us old-timers go through for a little quality time with our family. -  It means: "It's hard to bond with your family when you're an old man" ??
7.I created a homicidal pterodactyl-tron. - "tron"?
8.I had a run-in with the lake police! - He was fleeing from them?
9.an old man:"Come quick before he scramdoodles away!" ... Mabel:"He did use the word "scrapdoodle". scramdoodles, scrapdoodle - ??

Answer

Well, we have an expression, "By gum!"  So I guess "By gummity!" is a funny version of it.

Smitheroons is the writer's version of smithereens (very small pieces).

We have an expression "shimmy-shammy' and it sort of means to move in a funny way or to 
do some unscrupulous political dealings. There used to be a dance move called 'to shimmy.'

She misses (ah! how sad) me every time....  she..... tries to shoot me!  It's a joke, a play on words. Think about it!

Contrived - look it up in the dictionary!

"What lengths we go to....!"  What efforts we make!

-tron is a suffix often used in science. Positron, electron, cyclotron. A pterodactyl-tron is a made up word, and it sounds as if it might be a robot pterodactyl or a machine for generating pterodactyls! What fun! Oh, as it says it's a homicidal pterodactyl-tron it must be the former.

A run-in with the police! Oh dear. An encounter with very negative consequences. Probably ended up in jail!!!! Or maybe only with a speeding ticket.

We have the verb - to scram. Go away/run away  quickly. We have plenty of uses for 'doodle.' And 'scrap'. They are all English words. So 'scramdoodle' and 'scrapdoodle' are made up from the. To 'scramdoodle away' is probably to sort of scuttle away, or to scram. (to run off)

Many of these are playful words the writer has invented, or cobbled together from others. Lewis Carroll (Alice in Wonderland/Alice Through The Looking Glass) was a master of new words. Many actually became well established and are now in the dictionary.

Answer

1. Imaginary woord, a product of the writer
2. smithereens=small pieces
3. another imaginary word
4. This is a joke relying on the different definitions of the word "miss". A person with good aim can throw an object and hit the target.
5. forced or manipulated
6. Old-timers endure hardship to be with their families.
7. Imaginary construction
8. A run-in is an encounter, with a negative connotation.
9. an imaginary verb

Answer

Many of these words are not real English words.  They were invented by the writers because they sound funny, especially to children.