[admin@MikroTik] /ip firewall service-port> set gre disabled=no           
no such item

/ interface ethernet 
set ether1 name="ether1"

/ interface bridge 
add name="lan" arp=proxy-arp

/ interface bridge port 
add interface=ether1 bridge=lan

/ ip address 
add address=192.168.0.1/24 interface=lan

/ ip dns 
allow-remote-requests=yes

/ ip firewall service-port 
[color=#800000]set gre disabled=no [/color]
set pptp disabled=no 

/ ip pool 
add name="pptp" ranges=192.168.0.200-192.168.0.229 

/ ppp profile 
add name="pptp-in" local-address=192.168.0.1 remote-address=pptp use-encryption=required only-one=yes change-tcp-mss=yes dns-server=192.168.0.1

/ interface pptp-server server 
set enabled=yes max-mtu=1460 max-mru=1460 authentication=chap,mschap1,mschap2 default-profile=pptp-in 

/ ppp secret 
add name="user-1" service=pptp password="******" profile=pptp-in
add name="user-2" service=pptp password="******" profile=pptp-in
# ...

tcp connection established from x.x.x.x
<pptp-0>: waiting for call...
<pptp-0>: terminating...
<pptp-0>: disconnected