Question I have a set of solid state preamps which drives a power amp. These preamps are in a stomp box format. I use 3 preamps stompboxes, 1 for clean, 1 for dirty and 1 for lead. I don't use any traditional amplifier. Can I use the loops 1 to 4 (meant for effects pedal) for these 3 preamp inserts

Yes, it is possible to connect pre-amps in loops 1 to 4. These loops are designed for all kind of effects, drive and pre-amps devices.   The only difference between loop 1-4 and the insert loop is that the latter has pseudo-balanced connectors; this allows the use of longer ("balanced" or "balanced to unbalanced") cables to connect to a pre-amp or a head pre-amp, in the case the GFX01 remains under the board. This will reduce the hum usually produced by long cables.