OK have added a simple network picture to simplify my description.
3 sites A, B and C
GRE tunnel between A and B
GRE tunnel Between C and B
GRE tunnels works no problem!
on site B i have device x.x.x.10 i’m able to ping that device from site C via tunnel as well as all other devices on that network.
From site A from any PC I may ping any device on site B but not the device x.x.x.10 via GRE, x.x.x.10 ping timing out. If i ping x.x.x.10 from site A router itself ping is successful 
Did i mention what i may ping any other device? Yes I can ping any other devise on site B but not x.x.x.10
Where is the problem? cause I’m giving up already unless its something to do with ISP
Any smart ideas.?
And nothing special about x.x.x.10 in mikrotikB config?
No firewall/NAT?
On router B on NAT i have just a usual masquerada rule, and some port forward rules, on firewall just basic firewall nothing what would stop computers on A to ping specific device an site B even i can ping it from A router.
I suggest you to try to find the step on which the packet from A network to your mysterious host on B is lost:
use torch, packet sniffer or just create a couple of rules on top of the firewall forward chain both on A and B routers - one rule for way from A network to x.x.x.10, and one for way back.