If you have an LTC, you can take possession of them. This way, you won't have to worry about them wandering off.
If Uncle has Power of Attorney that is broad enough, Uncle can transfer them to you, in Gramp's name. That's the purpose of the Power of Attorney. Note - the PoA ends, when Gramps dies.
If Uncle wants $ for the guns, you can take them to a dealer (since you have the LTC), and Uncle can sell/consign them for Gramps.
If you want some or all, get them appraised, or use Gunbroker to get an idea of "fair" value, and buy them.
IANAL, but there are trickier firearms transfer problems.