What should I spend my GP on?

[AmusedDragon]

I've bought enough fire runes for 120 invention at this point James but I'm sure other people need them too.

 

As for what you should be doing...

 

10M is literally nothing this day in age in the game. You can easily make 1-5mil an hour doing decent slayer tasks and even mid-tier bossing. Especially with the current prices of stuff from GWD and barrows.

 

I'd recommend picking something and sticking with it. Even if you go 30 chests dry that doesn't mean you wont get 2 items in 1 chest later.

[James]

I'd honestly work on Slayer and maybe do like 30-60 minutes of barrows a day. You'll make a lot of money and improve your combat stats greatly while doing so. Once you're a higher level with appropriate gear we have a lot of members who can teach you higher level pvm. @Pearl recently made a gazillion GP off of farming Araxxor for a month. 

[Memphus]

Put it in your Miscellania Kingdom coffers. Then you don't even have to play the game, yet you are getting an awesome ROI.

[Veggie]

#1 priority, T80 weapon(s)

#2 priority, 95 Prayer so that you can have Soul Split and use it too (92 Prayer without 95 is worthless)

 

From there, you have most of the game unlocked in terms of things to spend money on. The rest of your money goes into a bottomless pit getting you meaningless levels (at least it helps towards max total) and slightly better stuff. Noxious scythe is the biggest upgrade you have left that you could get, but that's really expensive. T90 range and mage are unnecessary unless you're bossing, and bossing is unnecessary unless you're farming money for post-max XP or just to hog some money.