To my understanding all the gearbox or sporting penalties do not matter at all for 'back of the grid' starters,
chrisc90 you did miss Sainz getting another +5 which put him in that position, it's not because of the gearbox pen - but i see this has been cleared up whilst i was typing this
i'm positive that what i wrote here was correct
viewtopic.php?p=1087972#p1087972
Here's (again) the regs:
42.3 The Grid
a) [... normal quali positions...]
b) [... for those without a time set ...]
c) Once the grid has been established [...] grid position penalties will be applied to the drivers in question. [NOT BOTG]
i) The driver with the higher classification from the qualifying practice session will have precedence.
d) Once the grid has been established in accordance with Article 42.3a), 42.3b) and 42.3c), grid penalties for any driver required to start the race from the back of the grid after incurring a penalty under Article 28.3 will be applied.
i) If more than one (1) driver is required to start the race from the back of the grid they will be arranged in qualifying order.
Verstappen, Perez and Bottas will be in their quali position + penalty,
Sainz Hamilton and Tsunoda will be at the back of the grid, fighting for 18, 19, 20 - Yuki could even start ahead of those two despite having +13 via sporting penalties
They do not apply the penalties and then put them at the back of the grid and PU penalties are completely independent from the other ones.