In the global section of your dhcpd.conf, put the following (maybe a subset will suffice, I didn't try):

# Declare options needed for PXELinux
option space pxelinux;
option pxelinux.magic      code 208 = string;
option pxelinux.configfile code 209 = text;
option pxelinux.pathprefix code 210 = text;
option pxelinux.reboottime code 211 = unsigned integer 32;

#
# gPXE-specific encapsulated options
#
option space gpxe;
option gpxe-encap-opts code 175 = encapsulate gpxe;
option gpxe.priority code 1 = signed integer 8;
option gpxe.keep-san code 8 = unsigned integer 8;
option gpxe.no-pxedhcp code 176 = unsigned integer 8;
option gpxe.bus-id code 177 = string;
option gpxe.bios-drive code 189 = unsigned integer 8;
option gpxe.username code 190 = string;
option gpxe.password code 191 = string;
option gpxe.reverse-username code 192 = string;
option gpxe.reverse-password code 193 = string;
option gpxe.version code 235 = string;
# Other options that may be useful
#
option iscsi-initiator-iqn code 203 = string;
	

Inside a shared network, subnet or group in dhcpd.conf, put a snippet:

option pxelinux.pathprefix "/tftpboot/";
if exists dhcp-parameter-request-list { option dhcp-parameter-request-list =  concat( option dhcp-parameter-request-list,d0,d1,d2,d3); }
option pxelinux.reboottime 30;
option pxelinux.configfile "configs/lwppxe";
option pxelinux.magic f1:00:74:7e;
site-option-space "pxelinux";
filename "/tftpboot/pxelinux.0";
next-server 129.125.1.1;