It depends of many things.
1st before try to run the GPU after replacing the thermal pads,
I suggest after assembly it, then open it again (without trying to run the GPU)
and after open it check carefully the thermal pads for any imprints over each thermal pads that you have replaced,
and ensure a good imprints over each one of them.
+ Also check for a good imprints and make sure it has any imprints at all,
from the GPU core, look over GPU heastink where is suppose to make contact with GPU core of there is any imprint and for good imprint.
If there are good imprints over all places, then assembly it back and go try to run and test the GPU.
If there is no or missing imprints somewhere, then re-do the job in there until a good imprints is archived.
If there are good imprints, but you have a poor GPU core temp after that,
then reduce the thickness of the possible thermal pads which caused the issue and then test again.
Usually those are closest thermal pads near to the GPU core, i.e. thermal pads for the vram chips.
[but that may not be always the case, sometimes also other thermal pads also can lift the heatsink and prevent the perfect contact between GPU core and heatsink]