I think the radeon 6900 series has a max roof of 100c-105c if I'm not mistaken, so in that instance you should be fine at 77c. Anything over 85-90c would probably warrant a better aftermarket cooler and/or a reapplication of thermal paste for the heat sink just to be safe.
The processor is another story. I think 75c, especially judging by the hwmonito readings showing the cpu not even being at full load (disclamer: hwmonitor also has a problem of reading temperatures accurately from what I've seen, so your temps might not be accurate and could be off by a decent margin) could be a problem. I would think that a fresh coat of thermal paste and a decent aftermarket coolers could bring that temperature down by as much as 5-15C. It worked for me anyways. I was running at about 75c on my 4790k before I said, "Fuck it. It's time to reapply the thermal paste with my fumble fingers and "scared every second when I touch pc components mindset so just do it and get it over with" attitude, and my temperatures are around 65c-69c under full load when playing demanding games, so I must have done something right.
Will your cpu burn out over the next 6 hours? Probably not, but given my personal wariness and being super careful with owning a $2000+ computer (including monitor, keyboard and mouse costs of course) for the first time in OVER A DECADE, I stare at readouts and all the fancy numbers and readouts that various programs that display pretty much every facet of my computer's general status at any given time....I need to start playing more games and enjoy more of what my PC can offer in terms of overall entertainment.
is it à good price?