Cometic makes a good gasket kit for reasonable cost. The OEM gaskets are not too expensive.
Bearings are readily available as they cross over to many other bikes. All Balls Racing has crank bearing and crank seals in a kit or you can buy them seperately from a bearing house. Below are my notes for the bearing crossovers that I have found that work for the earlier bikes up to at least 2004. These bearings should cross over for your 2003 250 but since GG made changes on the fly, you may be best off opening it up and getting the same bearings that are in your engine now.
Boyesen Reeds Boyesen PSR-21 1996 YZ250 or RL-91 for the GasGas.
GasGas Transmission bearings 250 and 300
Bearing # Grainger #
Counter shaft bearing 6305C3 4ZXJ1
Countershaft Seal 30x40x7
Clutch bearing 6205Zc3E
Trans shafts 6203c3 4zxe8
01-02 Crank 63/28c3 or 63/28 maybe All Balls Part Number: 24-1112
1997-2000 63006 C3 This has a different clutch side crank seal too 30x52x8
Ignition crank roller Bearing, NJ206ET-C3 or KTM ? 50330082000 which has higher c3/4 tolerance
Note - that some engines (250 and 300) went from two ball bearings to a ball bearing on the clutch side and a roller bearing on the ignition side. The 2001 Xc250 had the roller bearing on the ignition side but the 2004 200 had two ball bearings.
6206c3 should be a ball bearing replacement for the roller bearing that was used in the previous GG bikes and almost all other common MX 2 strokes..
Same bearing as the Honda CR250 1990-2008, Honda CR500 1989-2002, Kawasaki KX250 1993-2008, Kawasaki KX500 1993-3005, Suzuki RM250 1999-2007, Yamaha YZ250 1989-2013
(I believe a previous GG member used these in their 07 300 -NJ206 ET -C3 (LHS) and 63/28-c3 (RHS))
question is are the Crank seals the same as the 01 02 models ? ie kok 20x35x6 -viton (LHS) and kok 38x52x7 -viton (RHS)
1 x SMALL END BEARING INA KBK 18x22x22
1 x LHS CRANK BEARING SKF NJ206ECP - C3 ( I think this should be the ET and not ECP)
1 x RHS CRANK BEARING SKF 63/28 - C3
1 x COUNTERSHAFT BEARING SKF 6305 - C3
1 x LHS CRANK SEAL KOK 20x35x6 ? VITON ktm 47030076100 04-14 105 85 65
1 x RHS CRANK SEAL KOK 38x52x7 - VITON ktm 0760385273 94-14 250 300
1 x COUNTERSHAFT SEAL 30x40x7 Suzuki 09283-30061
Shift Shaft Seal 12x22x7
KICKSTART shaft SEAL 20X30X7