There's a good guide explaining how to do No-NAT filtering on Multi WAN and Multi LAN with pfSense here: http://blog.martinshouse.com/2012/01/multi-wan-multi...
and as if it's any surprise, the blogger is using AAISP which would be the path of least resistance in terms of ISPs.
Irrespective of the Multi-WAN and LAN the blogger uses (which I expect you won't need), the transparent filtering via 2 ports in bridge mode on the router is exactly what you'd need.
If/when Gigaclear do allow you to bridge the device so their router is relegated to just modem duties, then pfSense can be reconfigured to handle the WAN directly - I don't know what Gigaclear use, but I expect it'd be PPPoE which pfSense can easily handle too, however the filtering doesn't change all that much.
In terms of the security related stuff, the thing you'll want is to use Suricata, which pfSense has as a package. This is quite a pain to set up (here's a video guide: https://www.youtube.com/watch?v=KRlbkG9Bh6I),
and will take a week or so of trial and error to 'bed in'. Security is never an 'on/off' switch.
Now while just about any hardware can handle 1gpbs WAN and routing, doing full IPS (intrusion prevention) is entirely a different matter in terms of performance.
To give an example, the £250 Ubiquiti USG Pro-4 is a great dual-WAN router and can easily handle 2x 1gbps WANs. However, for Suricata (IPS), it only boasts 250mbps of IPS throughput. The cheapo USG, only 85mbps. To get the full 1gpbs of IPS throughput, you need the £2,000 USG-XG, which otherwise does 8x 10GbE routing.
It'll be the same with pfSense: you'd need the £2,000 XG-1541 (same spec as the USG-XG) to do Suricata at 1gpbs. However, if you can compromise, I suggest getting one of the mid-range pfSense boxes like the XG-7100 or if you can find one, what I use, the SG-4860. These use the Atom C2000/C3000 processors which are more than adequate. I've not benchmarked the maximum speed of suricata on these machines, but I can say that it's not a bottleneck on 2x 80mbps FTTC lines. However, I did spend £500 for the privilege of having a proper firewall.
You can peruse all Ubiquiti and pfSense hardware at their UK distributors: https://www.msdist.co.uk/ubiquiti