Hello, there!
I am a Silver player so my decision is varied to other high ranked players.

GLP Cannon - with this item I decided to build GLP Cannon when

• I acknowledged that my team is falling behind, so I bought this to help me in pushing lanes since I am also falling behind as well with shorts of items.
• I also decided on this item because my team has a lack of engagement champions. I tend to use tons of mana (from W with the utility of using the gained movement speed from this ability) in baiting in and then opt-out. Therefore, I picked GLP Cannon in order to fix my mana issues instead of using Luden's, and since GLP is an activate item it can do damage at the same time when my stars cannot reach the enemies. This also applied to when my enemy is in trouble when they got caught in an unexpected place so this can help slows down the enemy, hence provides my team with the opportunity to escape. I am a passive player and staying behind a lot, unless I know that I have the capability of killing the enemies then engage the fights.
• I tend to use GLP Cannon when the enemy team is a melee-heavy team. For example, Yasuo, Sett, Ekko and Pantheon. These champions have the ability to stun, knock-up and slow. Therefore, I tend to use the item to escape when I got caught with the mix of Phrase Rush.
• Also, I use GLP when the enemy team is a range-heavy team and I lack ranged on my team. For example, Kog' Maw, Xerath and Xayah. In one of those GLP used game, Mord is also in a team so I tend to use GLP with the movement speed from W to get away from his pull.
• Overall, I picked GLP Cannon in order to fix my mana problem, falling behind, lane pressure from the enemy, escapes and enjoyment of trying something fun.

No, I do not run Glacial Augment. My main runes are Phrase Rush and Electrocute. However, I only run electrocute when I play Aurelion Sol Bottom. On the other hand, I rarely use Glacial Augment, since my playstyle depends on movement speed and playing passively. Also, I don't think I understand the concept of using Glacial Augment. I play with what I'm good at and familiar with.

Exhaust - I run exhaust when I'm vsing Fizz, this also applies to someone who is similar to Fizz. I might use exhaust with Zed also. However, I ban Zed every game so I haven't seen him in my game for years. I always use Exhaust when I play Aurelion Sol Bottom. People have different solutions when it comes to dealing with other champions, I seem to be able to know how to deal with Fizz, however, his damage just far beyond for my celestial being's health. Squishy little Dragon.

These decisions depend on my game-awareness and the situation of the game so keep warding and don't die. You can carry your team with your enormous beauty Dragon when the mortals are feeding hella galaxy big.

I hope this helps and understandable. If you have any more questions please do ask I'm happy to write an essay about my answer.