This simple rules of iptables enough for VPS/VDS Linux-based server for usage him as transparent proxy. Create new file for Bash script and put into this file the next text.

#!/bin/bash
echo "1"> /proc/sys/net/ipv4/ip_forward

PROXY="0.0.0.0"
SERVER="0.0.0.0"
PORT="7777"

iptables -F
iptables -t nat -A PREROUTING -p tcp --dst $PROXY --dport $PORT -j DNAT --to-destination $SERVER:$PORT
iptables -t nat -A POSTROUTING -p tcp --dst $SERVER --dport $PORT -j SNAT --to-source $PROXY

# Block ICMP traffic, e.g. ping.
iptables -t nat -A PREROUTING -p icmp --icmp-type echo-request --dst $PROXY -j DNAT --to-destination $SERVER
iptables -t nat -A POSTROUTING -p icmp --icmp-type echo-reply --dst $SERVER -j SNAT --to-source $PROXY

Change PROXY and SERVER variables by own. Add premissions for this file and execute.

For usage proxy gates you need to use the uLogin project.